Receiving device, semiconductor integrated circuit, transmitting/receiving device, transport apparatus portable transmitting/receiving device, communication system and receiving method

ABSTRACT

A receiving device includes a receiver circuit for outputting a received signal as a reception electric field intensity signal, an operation control circuit for controlling an operation of the receiver circuit, and an intermittent reception control circuit for outputting a periodic signal. A comparator circuit holds a first threshold indicating that the receiving device has entered a communication area and a second threshold indicating a start of a continuous electric field intensity measurement. If the reception electric field intensity signal is lower than the second threshold, the operation control circuit makes the receiver circuit perform intermittent operation. If the reception electric field intensity signal is equal to or higher than the second threshold, the operation control circuit makes the receiver circuit perform a continuous operation. If the reception electric field intensity signal is equal to or higher than the first threshold, the operation control circuit makes the demodulation circuit perform an operation.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. § 119(a) on JapanesePatent Application No. 2004-291981 filed on Oct. 4, 2004 the entirecontents of which are hereby incorporated by reference. Alsoincorporated by reference are the entire contents of Japanese PatentApplication No. 2005-179779 filed on Jun. 20, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a receiving device used for small areacommunication, represented by dedicated short range communication andwireless LAN communication, and a transmitting/receiving device, atransport apparatus and communication system including the receivingdevice.

2. Prior Art

FIG. 23 is a block diagram illustrating the configuration of knownvehicle-mounted communication system used for dedicated short rangecommunication and the like. The vehicle-mounted communication system ofFIG. 23 includes a control section 171 including a CPU 172 and a signalprocessing circuit 173, a transmitter 174, a circulator 175 connected toan antenna 176, a receiver 177 and an oscillator 178. Moreover, a powersupply circuit 179 for supplying a power supply voltage is connected tothe signal processing circuit 173, the CPU 172, the receiver 177 and theoscillator 178. A power supply voltage is supplied to the transmitter174 via a switch circuit (SW circuit) 180 (see, for example, JapaneseLaid-Open Publication No. 2000-278201).

Next, the operation of the known vehicle-mounted communication systemwill be further described.

FIG. 24 is a diagram illustrating the operation of the knownvehicle-mounted communication system. In an ETC (electric tollcollection) system or the like, when a vehicle with a vehicle-mountedcommunication system is outside of a communication area of an antenna ofa base station (roadside wireless equipment), a timer function of theCPU 172 is operated. When a predetermined time set in the timer haslapsed, the vehicle-mounted communication system is turned to a sleepmode. In the sleep mode, the CPU 172 and the oscillator 178 are stoppedby control of the signal processing circuit 173. At the same time, theswitch circuit 180 is turned OFF and a power supply to the transmitter174 is stopped.

In this state, when the vehicle with a vehicle-mounted communicationsystem approaches the communication area of the antenna of the basestation (roadside wireless equipment) and the antenna 176 of thevehicle-mounted communication system receives a call data signal fromthe base station (roadside wireless equipment), in response to the datasignal, the signal processing circuit 173, in response to the receiveddata signal, operates the oscillator 178 and the CPU 172 and also turnsON the switch circuit 180 to make the transmitter 174 supply power.Thus, the vehicle-mounted communication system becomes in an active modeand capable of transmission to the base station.

When transmission to the base station has been completed, thevehicle-mounted communication system becomes in a halt mode. In the haltmode, while the oscillator 178 stays in an operation state, onlyoperations of the CPU 172 and the transmitter 174 are stopped. When thevehicle-mounted communication system re-starts its operation, theoscillator 178 is in an operation and thus no waiting time is requiredto re-start an operation of the oscillator 178. Accordingly, thevehicle-mounted communication system can be returned to an active modefrom a halt mode.

In the known example, a halt mode is set between a sleep mode and anactive mode. Thus, power consumption in a non-communication state can besuppressed and also the vehicle-mounted communication system can quicklyreturn to an active mode in response to a request for re-connection, sothat a response speed with respect to a re-connection request can beincreased.

SUMMARY OF THE INVENTION

More and more functions are expected to be incorporated into acommunication system in future and thus reduction in power consumptionis strongly demanded. Specifically, development of a portable receivingdevice for receiving a signal transmitted from a base station isexpected. To achieve a portable receiving device, reduction in powerconsumption becomes more important than ever. However, in a known powerconsumption reduction technology for a vehicle-mounted communicationsystem disclosed in Japanese Laid-Open Publication No. 2000-278201, athreshold is provided for reception electric field intensity. Thus, itis a transmitter, an oscillator, a CPU or like circuit that can bestopped based on the threshold in a non-communication state, and othercircuits such as a receiver have to be ON all the time to judge whetheror not the vehicle-mounted communication system is within acommunication area of an antenna of a base station. Therefore, in theknown technology, power consumption of a receiver circuit can not bereduced.

It is therefore an object of the present invention to solve theabove-described problem and to provide a receiving device and acommunication system which allow reduction in power consumption in anon-communication state without reducing a speed of reception of asignal transmitted from a base station, and also provide a communicationmethod using the receiving device and the communication system.

To solve the above described problems, the present invention ischaracterized in that in a receiving device provided in a communicationsystem (moving station), two or more thresholds are set for a receptionelectric field intensity of an electric wave received by the receivingdevice.

Thus, only necessary circuits for communication are started up in turnusing a comparison result for the reception electric field intensityusing the thresholds, location information for the receiving device, themoving speed of the receiving device and the like, so that power supplyto unnecessary circuits are stopped when the receiving device does notcommunicate with a base station. Thus, power consumption can be reduced.

Specifically, the receiving device of the present invention is areceiving device for receiving a signal transmitted from a base stationand includes: a receiver circuit for outputting a reception electricfield intensity of a signal received via an antenna as a receptionelectric field intensity signal; a comparator circuit for comparing alevel of the reception electric field intensity signal output from thereceiver circuit using a first threshold indicating that the receivingdevice is in a communication area and a second threshold which is lowerthan the first threshold and indicates a continuous field measurementlevel and outputting a result of the comparison; an intermittentreception control circuit for outputting a periodic signal forintermittently operating the receiver circuit; and an operation controlcircuit for controlling an operation of the receiver circuit based on acomparison result of the comparator circuit. In the above-describedstructure, based on the comparison result of the comparator circuit, ifthe level of the reception electric field intensity signal is lower thanthe second threshold, the operation control circuit makes the receivercircuit perform intermittent operation based on the periodic signal andif the level of the reception electric field intensity signal is equalto or higher than the second threshold, the operation control circuitmakes the receiver circuit perform a continuous operation.

As described above, when the electric field intensity of a receivedsignal is weak, the receiver circuit is intermittently operated. Thus,compared to the case where the receiver circuit is continuouslyoperated, power consumption can be reduced. Furthermore, when the levelof the electric field intensity signal is equal to or higher than thesecond intensity, by operating the receiver circuit, a delay of thereceiving operation can be prevented.

Moreover, the receiver circuit further includes the function ofamplifying and outputting the received signal may further include ademodulator circuit for demodulating the amplified signal and outputtingthe signal as a modulation signal. In the structure including thedemodulator circuit, the operation control circuit preferably controlsan operation of the demodulator circuit. In such a case, the operationof the demodulator circuit can be appropriately stopped, so that powerconsumption for the power in the demodulator circuit can be reduced.

Moreover, in the above-described receiving device of the presentinvention, it is preferable that, based on the comparison result of thecomparator circuit, the operation control circuit operates thedemodulator circuit when the level of the reception electric fieldintensity signal exceeds the first threshold. Therefore, a receivedsignal can be demodulated without delay.

Furthermore, it is preferable that, based on the comparison result ofthe comparator circuit, the operation control circuit supplies a firstclock signal to the demodulator circuit when the level of the receptionelectric field intensity signal exceeds the first threshold. As has beendescribed, the operation control circuit can supply a first clock signalto the demodulator circuit. Thus, the present invention is effectivewhen the demodulator circuit is a digital circuit.

Moreover, it is preferable that the above-described receiving device ofthe present invention, a signal processing circuit for processing thedemodulation signal output from the demodulator circuit; and aprocessing unit for arithmetically processing the demodulation signalprocessed by the signal processing circuit. In this structure, based onthe comparison result of the comparator circuit, the operation controlcircuit supplies a second clock signal to the signal processing circuitand the processing unit to control the signal processing circuit and theprocessing unit when the level of the reception electric field intensitysignal is lower than the second threshold. Thus, with appropriatecontrol of the second clock signal by the operation control circuit, theoperations of the signal processing circuit and the processing unit canbe controlled. For example, when the level of a reception electric fieldintensity signal is the second threshold or less, supply of the secondclock signal is stopped. Thus, the operations of the signal processingcircuit and the processing unit can be stopped, so that powerconsumption of each circuit can be reduced.

Moreover, it is preferable that the second threshold has a firstboundary value which is smaller than the second threshold and a secondboundary value which is larger than the second threshold. In such acase, the receiver circuit can be shifted from an intermittent operationto a continuous operation when the level of the reception electric fieldintensity signal is increased to exceed the second boundary value. Onthe other hand, the receiver circuit can be shifted from a continuousoperation to an intermittent operation when the level of the receptionelectric field intensity signal is reduced to be equal to the firstboundary value. That is, the first boundary value and the secondboundary value can be defined as a power OFF judgment level of thereceiver circuit and a power ON judgment level of the receiver circuit,respectively. As has been described, by providing boundary values, i.e.,upper and lower limits of the second threshold, effects of noisecontained in the electric field intensity of a received signal on theoperation of the receiver circuit can be removed. Therefore, even whenthe reception electric field intensity signal fluctuates up and down dueto noise, the generation of an operating error of the receiver circuitcan be prevented and a stable operation of the receiver circuit can beperformed.

Moreover, the above-described receiving device of the present inventionis characterized by further including a speed detector for detecting amoving speed of the receiving device (or communication system) andoutputting the detected speed as a speed signal. Furthermore, it ispreferable that the comparator circuit changes the second thresholdaccording to the speed signal. Accordingly, it is possible to flexiblycope with change in the moving speed of the receiving device(communication system) output from the speed detector so that theoperation of the receiver circuit is not delayed. In such a structure,when the moving speed of the receiving device (communication system) isincreased, it is preferable that the comparator circuit changes thesecond threshold so that a difference between the first threshold andthe second threshold becomes larger. When the moving speed of thereceiving device (communication system) is high, the receiving deviceapproaches the base station in a short time. Thus, by reducing thesecond threshold, the receiver circuit can be shifted from anintermittent operation to a continuous operation at an earlier timepoint. Accordingly, the receiving device becomes in a communicationstandby state in an earlier time point, so that communication with thebase station can be started without delay. On the other hand, when themoving speed of the receiving device (communication system) is low, ittakes longer time for the receiving device to approach the base station.Thus, by increasing the second threshold, the cycle of an intermittentoperation of the receiver circuit can be relatively increased.Accordingly, when the moving speed of the receiving device(communication system) is low, the second threshold is increased, sothat power consumption of the receiver circuit can be reduced.

Moreover, in the above-described receiving device of the presentinvention, it is preferable that the intermittent reception controlcircuit changes the periodic signal according to the reception electricfield intensity signal. In such a structure, when the level of thereception electric field intensity signal becomes high, the intermittentcontrol circuit shortens a cycle of the periodic signal. Accordingly,the receiver circuit is operated at shorter intervals and it is possibleto cope with change in reception electric field intensity signal morequickly. Thus, a time point where the reception electric field intensityexceeds the second threshold can be judged with a reduced level oferrors, so that delay of the operation of the receiver circuit can beprevented.

Moreover, it is preferable that the receiving device of the presentinvention further includes a speed detector for detecting a moving speedof the receiving device (communication system) and outputting thedetected speed as a speed signal. Furthermore, in such a structure, itis preferable that the intermittent reception control circuit changesthe periodic signal according to the speed signal. Thus, theintermittent operation cycle of the receiver circuit can be changedaccording to change in the moving speed of the receiving device(communication system) and delay of the operation of the receivercircuit can be prevented. For example, in the above-described structure,when the moving speed of the receiving device (communication system) isincreased, a distance between the receiving device and the base stationis quickly reduced and the cycle of an intermittent operation can bereduced. Accordingly, when the moving speed of the receiving device ishigh, the receiver circuit is operated at short intervals, so that thereceiver circuit can be effectively shifted from an intermittentoperation to a continuous operation without delay.

Moreover, the receiving device of the present invention is characterizedby further including: a memory for storing location information for thebase station; and a location detector for detecting a location of thereceiving device. In this structure, it is preferable that theintermittent reception control circuit changes the periodic signalaccording to a result of comparison between the location of the basestation stored in the memory and the result of detection by the locationdetector. Thus, a distance between the base station and the receivingdevice is appropriately understood and the cycle of an intermittentoperation can be changed, so that delay of shift of the receiving deviceto a continuous operation can be prevented.

For example, it is preferable that the intermittent reception controlcircuit shortens a cycle of the intermittent operation of the receivingdevice when a distance between the base station and the receiving deviceis decreased. Thus, the receiver circuit can measure the receptionelectric field intensity at short intervals, so that the receivercircuit can be effectively shifted from an intermittent operation to acontinuous operation without delay.

Furthermore, in the above-described structure, it is preferable thatwhen the receiving device passes a new base station, the location of thenew base station is written in the memory. Thus, location informationfor a base station which has not been stored is newly stored in thememory and therefore, when the receiving device passes the base stationfor the second and subsequent times, the receiving device can recognizethe base station as an already known base station. Thus, delay of anoperation of the receiver circuit can be avoided.

Moreover, the receiving device of the present invention is characterizedby further including a reception control circuit for controlling areceiving time which it takes for the receiver circuit to receive thesignal via the antenna. In this structure, it is preferable that thecomparator circuit compares a level of the reception electric intensitysignal with, in addition to the first threshold and the secondthreshold, a third threshold which is lower than the second thresholdand indicates whether or not the signal to be received exists. In thiscase, it is preferable that the comparison result of the comparatorcircuit, when the level of the reception electric field intensity signalexceeds the third threshold, the reception control circuit increases thereceiving time so that the receiving time becomes longer than thereceiving time when the level of the reception electric field intensitysignal is equal to or lower than the third threshold. Thus, for example,even when signals of a plurality of communication frequencies arereceived, effects of other frequency signals can be removed.

Moreover, it is preferable that the intermittent reception controlcircuit increases a cycle of the periodic signal when the level of thereception electric field intensity signal is equal to or lower than thethird threshold.

Moreover, in the receiving device of the present invention, when signalsof a plurality of frequencies are received, the comparator circuitperforms comparison of each of the signals of a plurality of frequencieswith threshold. In such a case, the comparison result of the comparatorcircuit, when the level of the reception electric field intensity signalfor one of the signals of a plurality of frequencies exceeds the thirdthreshold, it is preferable that the reception control circuit increasesan operation time in an intermittence operation of the receiver circuitso that the operation time becomes longer than the operation time whenthe level of the reception electric field intensity signal is equal toor lower than the third threshold. Thus, a signal of the strongestfrequency can be received over a sufficient time for removing effects ofsignals of other frequencies. Therefore, a signal of a frequency that abase station uses can be reliably received.

Moreover, there may be cases where the receiving device of the presentinvention further includes a reception control circuit for controlling areceiving time of a received signal and signals of a plurality offrequencies are received. In such a case, the comparator circuitperforms comparison of each of the signals of a plurality offrequencies. The comparison result of the comparator circuit, when thelevel of one of the plurality of reception electric field intensitysignals exceeds the third threshold, it is preferable that the receptioncontrol circuit increases the receiving time of the receiver circuit forsignals of other frequencies so that the receiving time becomes longerthan the receiving time when each of the signals of a plurality offrequencies is equal to or lower than the third threshold. Thus, effectsof signals of other frequencies can be removed and a signal of afrequency transmitted from a base station can be reliably received.

Moreover, it is preferable that the receiving device of the presentinvention further includes a reception control circuit for measuring arate of change in the reception electric field intensity and thereception control circuit changes, according to the rate of change, areceiving time which it takes for the receiver circuit to receive thesignal. Thus, a signal to be received can be more reliably received.

Moreover, in the present invention, when signals of a plurality offrequencies are received, it is preferable that the comparator circuitperforms comparison of each of the signals of a plurality of frequencieswith threshold. Furthermore, when the rate of change in the receptionelectric filed intensity of one of signals of the plurality offrequencies is increased, an operation time of an intermittent operationof a receiver circuit for the frequency signal is increased, so that asignal of a frequency transmitted from a base station can be reliablyreceived.

Moreover, the receiving device of the present invention is characterizedby further including: a reception control circuit for controlling areceiving time which it takes for the receiver circuit to receive asignal via an antenna; a memory for storing a location of a base stationand frequency information for a signal transmitted from the basestation; and a location detector for detecting a location of thereceiving device. In this structure, it is preferable that the receptioncontrol circuit changes the receiving time according to a result ofcomparison between the location of the base station stored in the memoryand a detection result of the location detector. Thus, it becomespossible to keep a delay time of a start of receiving a signal smallwithout effects of noise or obstacles.

Moreover, according to the present invention, when the distance betweenthe base station and the receiving device is reduced, it is preferablethat the receiving time for a signal transmitted from the base stationis increased. Thus, delay of a start of receiving a signal can bereduced.

Moreover, it is preferable that the receiving device of the presentinvention further includes a station selector for selecting, fromsignals of a plurality of frequencies, a frequency signal which thereceiver circuit is to receive, based on an output of the receptioncontrol circuit. In this structure, as a result of a comparison of thecomparator circuit, the station selector can select, from the signals ofa plurality of frequencies, a frequency signal having a receptionelectric field intensity level exceeding the second threshold and thereceiver circuit can receive the selected frequency signal.

Moreover, it is preferable that a communication system is formed so asto include: the above-described receiving device; an antenna forreceiving a signal transmitted from a base station; and a transmittingdevice for outputting (transmitting) a signal input (received) via theantenna and processed to the base station via the antenna and be capableof transmitting/receiving a signal.

Moreover, it is preferable that the above-described communication systemof the present invention is integrated on a semiconductor substrate,thereby forming a semiconductor integrated circuit. Thus, atransmitting/receiving IC including a receiving device and atransmitting device, a control section including a signal processingcircuit, a CPU, and an antenna can be mounted on a chip.

Moreover, it is preferable that the receiving device is installed in atransport apparatus such as a car. In such a case, the communicationsystem can obtain power from a driving motor that the transportapparatus includes and, when the transport apparatus is moving,information can be transmitted/received to/from a fixed base stationwithout delay.

Moreover, the above-described receiving device may be used for atransportable wireless communication system or other communicationsystems.

Moreover, the receiving device of the present invention is characterizedby further including a notifying section for notifying, the comparisonresult of the comparator circuit, that the level of the receptionelectric field intensity signal has exceeded the second threshold. Forexample, in an ETC system, when the receiving device approaches atollgate, i.e., a base station, it is notified that the level of thereception electric field intensity signal has exceeded the secondthreshold, thereby preventing a failure of placing an external componentsuch as a pay card or the like. Therefore, exchange of information atthe tollgate (base station) can be reliably performed.

A receiving method according to the present invention is a method forreceiving a signal using a receiving device for receiving a signaltransmitted from a base station, and includes: a first step of receivingthe signal via an antenna by a receiver circuit; a second step ofoutputting a reception electric field intensity of the received signalas a reception electric field intensity signal from the receivercircuit; a third step of comparing the level of the reception electricfield intensity signal with a first threshold indicating that thereceiving device is in a communication area and a second threshold whichis lower than the first threshold and indicates a continuous fieldmeasurement start level; a fourth step of making, if the level of thereception electric field intensity signal is lower than the secondthreshold, as a result of the comparison, the receiver circuit performan intermittent operation; and a fifth step of making, if the level ofthe reception electric field intensity signal is equal to or higher thanthe second threshold, as a result of the comparison, the receivercircuit perform a continuous operation.

As described above, according to the method of the present invention,when the electric filed intensity of a received signal is weak, thereceiver circuit is made to perform an intermittent operation. Thus,compared to the case where the receiver circuit is performed for all thetime, power consumption can be reduced. Furthermore, when the level ofthe electric field intensity signal becomes equal to or higher than thesecond threshold, the receiver circuit is operated, so that delay of areception operation can be prevented.

Moreover, the receiving method of the present invention further includesa sixth step of making, if the level of the reception electric fieldintensity signal is equal to or higher than the first threshold, as aresult of the comparison, a demodulator circuit for demodulating thesignal received by the receiver circuit perform an operation.

Thus, when the receiving device enters a communication area, receptionof a signal can be started without delay. Moreover, when the level ofthe reception electric field intensity signal is lower than the firstthreshold, the operation of the demodulator circuit can be stopped. Insuch a case, power consumption at the power of the demodulator circuitcan be reduced.

Moreover, according to the present invention, it is preferable that inthe sixth step, a clock signal for demodulation operation is given tothe demodulator circuit. Thus, providing a clock signal to thedemodulator circuit is effective when the demodulator circuit is adigital circuit.

In the receiving method of the present invention, it is preferable thateach of the fourth step and the fifth step includes the step of giving aclock signal to a signal processing circuit for processing a signaloutput from the receiver circuit and a processing unit forarithmetically processing the signal.

It is preferable that the third step includes the step of comparing thelevel of the reception electric field intensity signal with the secondthreshold by hysterisis control using a first boundary value which islower than the second threshold and a second boundary value which ishigher than the second threshold. In such a case, when the receptionelectric field intension signal is increased and exceeds the secondboundary level, the receiver circuit can be shifted from an intermittentoperation to a continuous operation. On the other hand, when thereception electric field intensity signal is reduced and becomes equalto the first boundary level, the receiver circuit can be shifted from acontinuous operation to an intermittent operation. That is, the firstand second boundary values can be defined as the power OFF judgmentlevel of the receiver circuit and the power ON judgment level of thereceiver circuit, respectively. With the boundary values, i.e., upperand lower limits of the second threshold provided, effects of noisecontained in the electric field intensity of a received signal on anoperation of the receiver circuit can be removed. Therefore, even whenthe reception electric field intensity signal fluctuates due to noise,the occurrence of an operation error of the receiver circuit can beprevented and a stable operation of the receiver circuit can beprovided.

Moreover, it is preferable that the receiving method of the presentinvention includes before the third step: a seventh step of detecting amoving speed of the receiving device and outputting the moving speed asa speed signal; and an eighth step of changing the second thresholdaccording to the speed signal. Thus, it is possible to flexibly copewith change in the moving speed of the receiving device (communicationsystem) output from the speed detector so that the operation of thereceiver circuit is not delayed.

Moreover, according to the receiving method of the present invention, itis preferable that in the eighth step, when the moving speed isincreased, the second threshold is changed so that a difference betweenthe first threshold and the second threshold is increased. Thus, it ispossible to flexibly cope with change in the moving speed of thereceiving device (communication system) output from the speed detectorso that the operation of the receiver circuit is not delayed.

Moreover, according to the receiving method of the present invention, itis preferable that the fourth step includes a ninth step of changing acycle of the intermittent operation according to the reception electricfield intensity signal. According to the receiving method, in the ninthstep, when the level of the reception electric field intensity signal ishigh, the cycle of the intermittent operation can be shortened. Thus, atime point when the reception field intension exceeds the secondthreshold can be judged with reduced level of errors, so that delay ofan operation of the receiver circuit can be prevented.

Moreover, it is preferable that the receiving method of the presentinvention further includes: a tenth step of detecting a moving speed ofthe receiving device and outputting the detected moving speed as a speedsignal; and an eleventh step of changing a cycle of the intermittentoperation according to the speed signal. Thus, the intermittentoperation cycle of the receiver circuit can be changed according tochange in the moving speed of the receiving device (communicationsystem) and delay of the operation of the receiver circuit can beprevented.

Moreover, according to the receiving method of the present invention, itis preferable that in the eleventh step, when the moving speed isincreased, the cycle of the intermittent operation is shortened.Accordingly, when the moving speed of the receiving device is high, thereceiver circuit is operated at short intervals, so that the receivercircuit can be effectively shifted from an intermittent operation to acontinuous operation without delay.

The receiving method of the present invention is characterized byfurther including: the step of detecting a location of the receivingdevice; and the step of calculating a distance between a location of thebase station stored in the receiving device and the detected location ofthe receiving device. In this case, it is preferable that the fourthstep further includes a twelfth step of changing a cycle of theintermittent operation according to the calculated distance. Thus, adistance between the base station and the receiving device isappropriately understood and the cycle of an intermittent operation canbe changed, so that delay of shift of the receiving device to acontinuous operation can be prevented.

Moreover, according to the above-described receiving method of thepresent invention, it is preferable that in the twelfth step, when thedistance between the base station and the receiving device is reduced,the cycle of the intermittent operation is shortened.

Moreover, according to the receiving method of the present invention,the third step further includes the step of comparing the level of thereception electric field intensity signal with a third threshold whichindicates whether or not a signal to be received exists and is lowerthan the second threshold. In this case, it is preferable that thefourth step includes the step of increasing, if the level of thereception electric field intensity signal exceeds the third threshold,as a result of the comparison, a receiving time of the receiver circuitso that the receiving time is longer than a receiving time when thelevel of the reception electric field intensity signal is equal to orlower than the third threshold. Thus, for example, even when signals ofa plurality of frequencies are received, effects of signals of otherfrequencies can be removed.

Moreover, according to the receiving method of the present invention, itis preferable that the fourth step includes the step of changing areceiving time which it takes for the receiver circuit to receive thesignal, according to a rate of change in the reception electric fieldintensity. Thus, reception of a signal to be received can be morereliably performed.

Moreover, the above-described receiving method of the present inventionmay further include: the step of detecting a location of the receivingdevice; and the step of calculating a distance between a location of thebase station stored in the receiving device and the detected location ofthe receiving device. In this case, it is preferable that the fourthstep includes the step of changing a receiving time which it takes forthe receiver circuit to receive the signal according to the calculateddistance. Thus, a delay time of a start of receiving signal can be keptsmall without being influenced by noise or obstacles.

It is preferable that the receiving method of the present inventionfurther includes, after the third step, the step of selecting, from theplurality of received signals, a signal with a reception electric fieldintensity signal having a level equal to or higher than the secondthreshold.

Moreover, according to the receiving method of the present invention, itis preferable that the fifth step includes the step of notifying thatthe level of the reception electric field intensity signal is equal toor higher than the second threshold. For example, in an ETC system, whenthe receiving device approaches a tollgate, i.e., a base station, it isnotified that the level of the reception electric field intensity signalhas exceeded the second threshold, thereby preventing a failure ofplacing an external component such as a pay card or the like. Thus,exchange information at the tollgate can be reliably performed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the configuration of an exemplaryreceiving device according to a first embodiment of the presentinvention.

FIG. 2 is a diagram illustrating a method for controlling the receivingdevice according to the first embodiment of the present invention.

FIG. 3 is a diagram illustrating a method for judging a reception startlevel in a receiving device according to a second embodiment of thepresent invention.

FIG. 4 is a block diagram illustrating the configuration of a receivingdevice according to a third embodiment of the present invention.

FIG. 5 is a diagram illustrating a method for controlling the receivingdevice according to the third embodiment of the present invention.

FIG. 6 is a block diagram illustrating the configuration of a receivingdevice according to a fourth embodiment of the present invention.

FIG. 7 is a diagram illustrating a method for controlling the receivingdevice according to the fourth embodiment of the present invention.

FIG. 8 is a block diagram illustrating the configuration of a receivingdevice according to a fifth embodiment of the present invention.

FIG. 9 is a diagram illustrating a method for controlling the receivingdevice according to the fifth embodiment of the present invention.

FIG. 10 is a block diagram illustrating the configuration of a receivingdevice according to a sixth embodiment of the present invention.

FIG. 11 is a diagram illustrating a method for controlling the receivingdevice according to the sixth embodiment of the present invention.

FIG. 12 is a block diagram illustrating the configuration of a receivingdevice according to a seventh embodiment of the present invention.

FIG. 13 is a diagram illustrating a method for controlling the receivingdevice according to the seventh embodiment of the present invention.

FIG. 14 is a diagram illustrating another method for controlling thereceiving device according to the seventh embodiment of the presentinvention.

FIG. 15 is a block diagram illustrating the configuration of a receivingdevice according to an eighth embodiment of the present invention.

FIG. 16 is a diagram illustrating a method for controlling the receivingdevice according to the eighth embodiment of the present invention.

FIG. 17 is a block diagram illustrating the configuration of a receivingdevice according to a ninth embodiment of the present invention.

FIG. 18 is a diagram illustrating a method for controlling the receivingdevice according to the ninth embodiment of the present invention.

FIG. 19 is a circuit diagram of a semiconductor integrated circuit for areceiving device according to a tenth embodiment of the presentinvention.

FIG. 20 is an illustration of a transport apparatus according to athirteenth embodiment of the present invention.

FIGS. 21A and 21B are illustrations describing a transportable wirelesscommunication system according to a fourteenth embodiment of the presentinvention.

FIG. 22 is an illustration of an example for transmitting/receivingdevice and transport apparatus according to a sixteenth embodiment ofthe present invention.

FIG. 23 is a block diagram illustrating the configuration of a knowncommunication system used for dedicated short range communication andthe like.

FIG. 24 is a diagram illustrating the operation of the knownvehicle-mounted communication system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of a receiving device according to the presentinvention will be described with reference to the accompanying drawings.

First Embodiment

FIG. 1 is a block diagram illustrating an exemplary configuration of areceiving device according to a first embodiment of the presentinvention. As shown in FIG. 1, the receiving device of this embodimentincludes a receiver circuit 1 for receiving a signal transmitted from abase station (not shown) via an antenna 7 and outputting an electricfield intensity of a received signal as a reception electric fieldintensity signal and a demodulator circuit 2 for demodulating the signalprocessed by the receiver circuit 1. Moreover, the receiving device ofthis embodiment also includes a comparator circuit 3 for comparing alevel of the reception electric field intensity signal output from thereceiver circuit 1 to a first threshold and a second threshold andmaking a judgment and an intermittent reception control circuit 5 foroutputting a signal for intermittently operating the receiver circuit 1.In this case, the intermittent reception control circuit 5, in otherwords, outputs an intermittent cycle signal for making the receivercircuit 1 intermittently receive a reception electric field intensitysignal. Furthermore, the receiving device of this embodiment includes anoperation control circuit 4 for controlling supply of power supplyvoltage to the receiver circuit 1 and the demodulator circuit 2, ajudging circuit 8 for controlling an output of the operation controlcircuit 4 according to a comparison result by the comparator circuit 3,a signal processing circuit 9 and a CPU (processing unit) 10 each forprocessing a demodulation signal output from the demodulator circuit 2.Note that the judging circuit 8 is formed of a logic circuit or thelike.

In the receiving device, when it is judged that the level of a receptionelectric field intensity signal is equal to or lower than the secondthreshold in the comparator circuit 3, the operation control circuit 4intermittently supplies power to the receiver circuit 1 according to anoutput from the intermittent reception control circuit 5. The secondthreshold will be described later.

Moreover, in the comparator circuit 3, when it is judged that the levelof a reception electric field intensity signal exceeds the secondthreshold, the operation control circuit 4 performs control so thatpower is supplied to the receiver circuit 1 at all the time regardlessof an output of the intermittent reception control circuit 5. In thiscase, the judging circuit 8 judges which of intermittent reception orcontinuous reception the receiver circuit 1 is to perform, based on ajudgment result of the comparator circuit 3 and an intermittent cyclesignal of the intermittent reception control circuit 5, and then outputsa result of the judgment to the operation control circuit 4.

Thus, the receiving device of this embodiment is characterized in thatthe intermittent reception control circuit 5 is provided therein andintermittent operation of the receiver circuit 1 is effectively adopted.In this case, the first threshold indicates a communication start level(reception start level) for communication with a base station and thesecond threshold indicates a continuous electric field intensitymeasurement start level (receiver circuit's operation start level).

For example, a receiving device according to the present invention isused for a communication system for a mobile station such as an ETC(electric toll collection) system or a DRSC (dedicated short rangecommunication) system, which receives a signal containing informationtransmitted from a base station or processes a received signal andtransmits the processed signal to a base station. The mobile station isinstalled in a vehicle or carried by a person and is communicable when adistance between the base station and the mobile station is within arange of an approximately 20 m or less.

Next, the operation of the receiving device will be described withreference to FIGS. 1 and 2. FIG. 2 is an illustration describing amethod for controlling the receiving device of FIG. 1. In FIG. 2, theabscissa indicates a moving distance (which will be herein referred toas a “time point t”) in the case where a communication system is assumedto be moving at a constant speed, and the ordinate indicates thereception electric field intensity of a received signal at a time pointt. Also, in FIG. 2, s1 is a demodulation signal output by thedemodulator circuit 2 of FIG. 1, s2 is a signal (power supply voltage)supplied to the receiver circuit 1 by the operation control circuit 4,s3 is a signal (power supply voltage) supplied to the demodulatorcircuit 2 by the operation control circuit 4, and s4 is a receptionelectric field intensity signal 25 output to the comparator circuit 3 bythe receiver circuit 1. In FIG. 2, for simplification, a reception filedintensity signal 25 when the receiving device (moving station) of thisembodiment approaches a base station. Hereinafter, the case where thelevel of the reception electric field intensity signal 25 is increasedin proportion to time in this embodiment will be described. A receptionelectric field intensity signal fluctuates not linearly but up and down.Note that this embodiment is nothing more than an example of the presentinvention and may be applied to a reception electric field intensitysignal in some other form.

Moreover, as shown in FIG. 2, in the demodulation signal s1, the signalsegment 20 is part in which information to a mobile station (i.e., atarget reception device) is contained and the signal segment 21 is partin which information to the mobile station is not contained. Needless tosay, the demodulation signal output s1 is not limited to the segments 20and 21 of the demodulation signal.

In this embodiment, first, when the receiving device is far from a basestation (before a time point t1), i.e., when it is judged by thecomparator circuit 3 that a reception electric field intensity v issmaller than the second threshold 27 (continuous electric fieldintensity measurement start level v2), the receiver circuit 1 is turnedON/OFF at regular intervals in the signal segment 22 (intermittentreception segment) of FIG. 2. In this case, the comparator circuit 3measures the reception electric field intensity v of a signal of acommunication frequency while the receiver circuit 1 is ON (intermittentreception segment). That is, in the signal segment 22, the receptionelectric field intensity is intermittently measured. During the signalsegment 22, power supply to the demodulator circuit 2 is stopped andalso operations of the CPU 10 and the signal processing circuit 9 arestopped or the CPU 10 and the signal processing circuit 9 are operatedat low speed by a clock.

Furthermore, when the receiving device approaches the base station, thereception electric field intensity v is increased with time (distance).Then, when the level of the reception filed intensity v exceeds thesecond threshold 27 (continuous electric field intensity measurementstart level v2), at the time point t1, the comparator circuit 3 judgesthat the receiving device 1 approaches a communication area andcontinuously supplies power to the receiver circuit 1 as in the receivercircuit power ON segment 23. Thus, the receiving device 1 is changedfrom an intermittent reception state (intermittent reception segment,i.e., the signal segment 22) to a continuous reception state (continuousreception segment, i.e., the receiver circuit power ON segment 23) andcontinuously measures the reception electric field intensity v. At thispoint of time, power supply to the demodulator circuit 2 is not startedand the signal processing circuit 9 for processing a demodulation signalis not operated. On the other hand, the CPU 10 is not operated or in alow-speed operation state.

Subsequently, when the receiving device further approaches the basestation and the reception electric field intensity v exceeds the firstthreshold 26 (reception start level v1) at the time point t2, thecomparator circuit 3 judges that the receiving device has entered acommunication area. Thus, the power of the demodulator circuit 2 and thepower of the signal processing circuit 9 are, then, turned ON by theoperation control circuit 4, and the CPU 10 becomes in an operationstate. Moreover, at this time point t2, the demodulator circuit 2becomes in a power ON segment 24. Thus, the signal input from theantenna 7 to the demodulator circuit 2 via the receiver circuit 1 isoutput as the demodulation signal s1. Then, the demodulator signal s1 inthe signal segment 20 is processed by the signal processing circuit 9.

In this embodiment, a value for the first threshold 26 is larger thanthe second threshold 27. Each of the first threshold 26 and the secondthreshold 27 is stored in the comparator circuit 3.

With the receiving device of this embodiment, when the receiving device(moving station) is far from a base station, the receiver circuit 1intermittently receives a signal and the power of the demodulatorcircuit 2 is turned OFF. Thus, compared to a known example shown inFIGS. 23 and 24, power consumption can be reduced in the receivercircuit 1 and the demodulator circuit 2. That is, power consumption ofthe receiving device in a standby status, for example, when thereceiving device is out of a communication area of a base station can bereduced. Therefore, the receiving device of this embodiment can bepreferably used not only in a vehicle-mounted communication system usinga dedicated short range communication system or a wireless LAN systembut also a portable transmitting/receiving device of which powerconsumption is required to be reduced.

Specifically, a vehicle-mounted transmitting/receiving device such as anETC system takes power directly from a car body (battery) in aconventional manner. Thus, power can be supplied to the vehicle-mountedtransmitting/receiving device at all the time and there has been no needto reduce power consumption. However, conventionally, there have beencases where a temperature in a car sometimes becomes higher than roomtemperature and, if the power of the transmitting/receiving device is ONat all the time in such an environment, the temperature of thetransmitting/receiving device is increased to a high temperature.According to the present invention, however, with reduction in powerconsumption, heating of the transmitting/receiving device can besuppressed and the effect of improving heat resistant reliability of thetransmitting/receiving device can be achieved.

Moreover, a threshold (the second threshold 27) for turning ON thereceiver circuit 1 is set at a different value from a value for athreshold (the first threshold 26) for turning ON the demodulatorcircuit 2. Thus, the power of the receiver circuit 1 can be turned ONprior to the demodulator circuit 2, the receiving device can be speedilystarted up when the receiving device approaches a communication area andthe receiving device can start communication with the base station.Accordingly, for example, if the receiving device of this embodiment isused in a transmitting/receiving device for an ETC system, the receivingdevice is operated without delay before a tollgate. Therefore, when acar approaches a tollgate, start of receiving a signal is delayed can beprevented and, therefore, an accident such as a crash of the car into anopen/close bar of the tollgate.

With use of a digital circuit for the receiver circuit 1 or thedemodulator circuit 2, even if a clock signal is controlled, instead ofpower supply source, the same effect can be achieved, so that powerconsumption can be reduced. In such a case, instead of the operationcontrol circuit 4 for controlling power supply, an operation controlcircuit for controlling clock supply to the receiver circuit 1, thedemodulator circuit 2, the signal processing circuit 9 and the CPU 10 isprovided.

The receiving device of this embodiment or a transport apparatus (suchas an ETC system-mounted vehicle) in which the receiving device of thisembodiment is mounted may include a notifying section for issuingwarning by a sound or a color when the reception electric fieldintensity v exceeds the second threshold 27. For example, when a soundis used for warning, music, a beeper or anything making a sound can beused. When a color is used for warning, light emitting diode, a lamp oranything having a display function may be used. Thus, a failure ofplacing an external component such as an ETC card necessary forcommunication operation with a base station can be prevented before avehicle enters a tollgate.

Also, even if a CPU, instead of the judging circuit 8, is used, the samefunction as that of the receiving device of FIG. 1 can be achieved.

Hereinafter, a setting standard for each of the first and the secondthresholds described above will be described.

The first threshold level:

The standard sensitivity of an ETC (ASK) defined by the 3. 4. 3. 2reception sensitivity of an ARIB STD-T75 receiver is −60 dBm. When asignal of −60 dBm or more is input, the signal has to be properlyreceived as a signal from a base station.

Also, as for no-response input standard, there is a rule of banningresponse to an electric wave of −70.5 dBme.i.r.p. or less. Accordingly,in assumption that an antenna of about 0 dBi is used, an allowable lowerlimit of the first threshold level is about −70 dBm for an input level.

Therefore, the first threshold level is set to be between theno-response input standard and the standard reception sensitivity. Inthis case, for example, the first threshold is in a range of −70 dBm ormore and −60 dBm or less.

Second threshold level:

The second threshold level is preferably an upper limit value (−70 dBm)or less of the no-response input level defined by the above-describedstandard and is set to be in a range of about −100 dBm, i.e., the levelof thermal noise in an antenna terminal of a receiver.

For example, as for a receiver with a noise figure is 10 dB, a noiselevel when no input is received is a level which is 10 dB larger thanthermal noise. Thus, in this case, the second threshold is preferablyset to be −90 dBm or more and −70 dBm or less. In this embodiment, thesecond threshold level is in a range of −90 dBm or more and −70 dBm orless.

Note that the settings for the first and second thresholds are appliedto each of the following embodiments.

Second Embodiment

As a second embodiment of the present invention, a hysteresis controlused in a receiving device according each embodiment of the presentinvention will be described.

FIG. 3 is a diagram illustrating a method for judging a reception startlevel v2 (second threshold 27) in a receiving device according to thesecond embodiment. As shown in FIG. 3, a signal received by the receivercircuit 1 contains noise and the like and the reception electric fieldintensity v of the received signal varies depending on a measuring timepoint. Thus, in the receiving device of FIG. 1, if the comparatorcircuit 3 judges the level of the reception electric field intensitysignal 25 based on the second threshold of a single value, there mightbe cases where an accurate operation of the receiver circuit 1 can notbe performed. That is, because of the influence of noise, there might becases where with a threshold assumed as a boundary, the receptionelectric field intensity v fluctuates up and down and unintentionalrepetition of turning ON/OFF of the receiver circuit is caused.

To cope with the above-described problem as shown in FIG. 3, a receivercircuit power ON judgment level (second boundary value 27A) which ishigher than the second threshold 27 and a receiver circuit power OFFjudgment level (first boundary value 27B) which is lower than the secondthreshold are stored in the comparator circuit 3, and the level of thereception electric field intensity signal 25 is judged with the receivercircuit power ON judgment level and the receiver circuit power OFFjudgment level assumed to be the second threshold 27. Specifically, whenthe level of the reception electric field intensity signal 25 exceedsthe second boundary value 27A, the power of the receiver circuit 1 isturned ON, and when the level of the reception electric field intensitysignal 25 becomes equal to or lower than first boundary value 27B, powersupply (or clock supply in the case of a digital circuit) to thereceiver circuit 1 is stopped.

If a difference between the second boundary value 27A and the firstboundary value 27B is properly set according to an oscillation width ofa received signal, a judgment error can be prevented in theabove-described manner. Note that in the same manner, a demodulatorcircuit power ON judgment level and a demodulator circuit power OFFjudgment level can be set for the first threshold 26 and an operation ofthe demodulator circuit 2 can be started without causing errors.

As has been described, in the control method of this embodiment, toprevent an operation error of the receiver circuit and the like, effectsof noise contained in an electric intensity signal of a received signalare removed using the first boundary value and the second boundaryvalue. The second boundary value is larger than the first boundary valueand can be the same value as the second threshold value. Moreover, it ispreferable that the first boundary value is set to be equal to or lowerthan a value obtained by subtracting, from the second threshold, amaximum value of amplitude of a reception electric filed intensitycontaining noise. For example, the first boundary value is preferablyset to be a value in the range from the second threshold to 6 dB orless. However, if a noise signal level when the receiving devicereceives no signal is −90 dBm, the first boundary value is set to be avalue which can be distinguished to be larger than thermal noise. Forexample, the first boundary value is preferably set to be between a 6 dBlower than the second threshold and −90 dBm.

Third Embodiment

Hereinafter, a receiving device according to a third embodiment of thepresent invention will be described with reference to FIG. 4. Note thatdetail also described in the first embodiment and the detail descriptionof each member also shown in FIG. 1 will be omitted.

FIG. 4 is a block diagram illustrating the configuration of thereceiving device of the third embodiment of the present invention. Asshown in FIG. 4, the receiving device of this embodiment includes areceiver circuit 1 for outputting a signal received via an antenna 7 asa reception filed intensity signal, a demodulator circuit 2 fordemodulating the signal processed by the receiver circuit 1 andoutputting the signal, a comparator circuit 3 for comparing the level ofthe reception electric field intensity signal output from the receivercircuit 1 with a first threshold and a second threshold and judging thelevel of the reception electric field intensity signal, an intermittentreception control circuit 5 for outputting a signal (intermittent cyclesignal) for intermittently measuring the reception electric fieldintensity signal, and an operation control circuit 4 for controllingsupply of power supply voltage to the receiver circuit 1 and thedemodulator circuit 2. Moreover, the receiving device of this embodimentfurther includes a judging circuit 8 for judging a method forcontrolling the operation control circuit 4 according to a judgmentresult of the comparator circuit 3, a signal processing circuit 9 and aCPU 10 each for processing a demodulation signal output from thedemodulator circuit 2, and a speed detector 31 for detecting a movingspeed of the receiving device and outputting a detection result to thecomparator circuit 3. As described above, the receiving device of thisembodiment has a configuration in which the speed detector 31 fordetecting a moving speed of the receiving device, i.e., a mobile stationis further provided in the receiving device of the first embodiment.

The operation of the receiving device will be described with referenceto FIGS. 4 and 5. FIG. 5 is a diagram illustrating a method forcontrolling the receiving device of FIG. 4. In FIG. 5, a signal segment20 of a demodulation signal output is part in which information to amoving station and a signal segment 21 of the demodulation signal ispart in which information to the moving station is not contained. Also,in FIG. 5, s1 is a demodulation signal output by the demodulator circuit2 of FIG. 1, s2 is a signal (power supply voltage) supplied to thereceiver circuit 1 by the operation control circuit 4, and s3 is asignal (power supply voltage) supplied to the demodulator circuit 2 bythe operation control circuit 4. Also, in FIG. 5, s4 is the receptionelectric field intensity signal 25 output to the comparator circuit 3 bythe receiver circuit 1.

As will be described below, the receiving device of this embodiment ischaracterized in that the second threshold 27 shown in the firstembodiment is changed depending on the moving speed of the receivingdevice detected by the speed detector 31. In FIG. 5, an example in whichfor the second threshold 27, a second threshold 43 is set in the casewhere the moving speed of the receiving device is low and a secondthreshold 44 is set in the case where the moving speed of the receivingdevice is high is shown. In this case, the second threshold 43 (secondthreshold level v2) is higher than the second threshold 44 (secondthreshold level v3).

First, when the receiving device approaches a base station from theoutside of a communication area of the base station, as the intermittentreception segment (signal segment 22) of FIG. 5 shows, the intermittentreception control circuit 5 periodically outputs an intermittent cyclesignal for turning ON/OFF the power of the receiver circuit 1 and thereceiver circuit 1 is controlled by the operation control circuit 4. Atthis time, the comparator circuit 3 measures a reception filed intensityv of a signal of a communication frequency while the receiver circuit 1is in an ON state. Moreover, as the receiving device approaches the basestation, the reception electric field intensity signal v is increased.For example, the level of a reception electric field intensity signal ischanged as the reception electric field intensity signal 25 of FIG. 5.

In this case, if the speed detector 31 measures the moving speed of thereceiving device and the obtained moving speed is a certain speed ormore, according to the measurement result, the second threshold 27stored in the comparator circuit 3 is lowered and the second threshold44 is used. If the obtained moving speed is the certain speed or less,the second threshold 27 stored in the comparator circuit 3 is increasedand the second threshold 43 is used.

In this case, the speed described above is, in general, a speed at whicha vehicle can safely travel under a regulation speed outside of acommunication area. For example, if the vehicle is on a highway, thespeed is set to be 80 km or less.

Accordingly, when the moving speed is high and the reception electricfield intensity v exceeds the second threshold level v3 at a time pointt3 which is earlier than a time point t1, the receiver circuit 1 isturned to be in a receiver circuit power ON segment (continuousreception segment) 42, i.e., a continuous reception state, from theintermittent reception state (intermittent reception segment, i.e.,signal segment 22).

On the other hand, when the moving speed is low and the receptionelectric field intensity v exceeds the second threshold level v2 at thetime point t1, the receiver circuit 1 is turned to be in the receivercircuit power ON segment (continuous reception segment) 41, i.e., acontinuous reception state, from the intermittent reception state(intermittent reception segment, i.e., signal segment 22), andcontinuously measures the reception electric field intensity v. Notethat at this point of time, power supply to the demodulator circuit 2 isnot started and the signal processing circuit 9 and the CPU (processingunit) 10 each for processing a demodulation signal are not operated.

Subsequently, at a time point t2, when the reception electric fieldintensity v exceeds the first threshold level v1, based on an output ofa judgment result of the comparator circuit 3, the judging circuit 8judges that the receiving device has entered the communication area.According to the judgment result of the judging circuit 8, the operationcontrol circuit 4 starts supply of power supply voltage to thedemodulator circuit 2 and is turned to be in a demodulator circuit powerON segment 24. Accordingly, the received signal input into thedemodulator circuit 2 from the antenna 7 via the receiver circuit 1 isoutput as a demodulation signal s1. Then, the demodulation signal s1 isprocessed by the signal processing circuit 9 and the CPU 10.

In the known receiving device, if the moving speed of the receivingdevice is high, the receiving device might travel a long distance duringa sleep mode and a start of receiving a signal might be delayed. Incontrast, in the receiving device of this embodiment, if the movingspeed of the receiving device is increased, the power of the receivercircuit 1 is turned ON at an earlier time. Thus, switching from theintermittent reception segment 22 to the continuous reception segment(the receiver circuit power ON segment 42) can be stably performed.Therefore, delay in a start of an operation of the receiver circuit 1can be prevented. Moreover, the receiver circuit 1 performs intermittentreception, thus reducing the amount of current to be consumed when thereceiving device is outside of a communication area and in a standbystatus.

When a digital circuit is used for the receiver circuit 1 or thedemodulator circuit 2, even if a clock signal is controlled, instead ofpower supply source, the same effect can be achieved, so that powerconsumption can be reduced.

In this embodiment, the case where the first threshold is higher thanthe second threshold has been described. However, as long as thereception electric field intensity signal is changed according to eachtime point for the receiving device, the first threshold and the secondthreshold may be set in a different manner. In such a case, when themoving speed of the receiving device is high, the first threshold andthe second threshold are changed so that a difference between the firstthreshold and the second threshold becomes larger.

As described above, with the receiving device of this embodiment, theelectric field intensity measurement start level (second threshold) ischanged according to the moving speed of the receiving device. Thus, thepower of the receiver circuit can be turned ON at a proper time withoutdelay.

Note that in this embodiment, the moving speed of the receiving devicemay be judged, for example, based on increase and decrease inacceleration of the receiving device. Specifically, assume that areceiving device is traveling at a certain speed. If it is found that amoving speed of the receiving device is increased, a threshold isreduced. If it is found that a moving speed of the receiving device isreduced, a threshold is increased. In this manner, even if a device formeasuring the moving speed of the receiving device is not installed toan axle shaft or the like, an acceleration sensor is used to obtain themoving speed of the receiving device. Therefore, an interconnect becomesunnecessary and a speed of a ship or the like, which do not include anaxis shaft can be measured.

Fourth Embodiment

FIG. 6 is a block diagram illustrating a receiving device according to afourth embodiment of the present invention. As shown in FIG. 6, thereceiving device of this embodiment includes a receiver circuit 1 foroutputting a signal received via an antenna 7 as a reception electricfield intensity signal, a demodulator circuit 2 for demodulating thesignal processed by the receiver circuit 1 and outputting the signal,and a comparator circuit 3 for comparing the level of the receptionelectric field intensity signal output from the receiver circuit 1 witha first threshold and a second threshold. Moreover, the receiving deviceof this embodiment further includes an intermittent reception controlcircuit 5 for outputting a signal for intermittently measuring thereception electric field intensity signal and changing a measuring cycleaccording to the intensity of the reception electric field intensitysignal, an operation control circuit 4 for controlling supply of powersupply voltage to the receiver circuit 1 and the demodulator circuit 2,a judging circuit 8 for changing control of the operation controlcircuit 4 according to a judgment result of the comparator circuit 3, asignal processing circuit 9 and a CPU 10 each for processing ademodulation signal output from the demodulator circuit 2.

Unlike the receiving device of the first embodiment, the receivingdevice of this embodiment is characterized in that an intermittent cyclein an intermittent cycle signal output from the intermittent receptioncontrol circuit 5 can be set according to a reception electric fieldintensity signal.

The operation of the receiving device of this embodiment will bedescribed with reference to FIGS. 6 and 7. FIG. 7 is a diagramillustrating a method for controlling the receiving device of FIG. 6.Note that the description of each member also shown in FIG. 2 and detailalso described in the first embodiment will be omitted.

In the receiving device of this embodiment, when the receiving device isoutside of a communication area and the level of the reception electricfield intensity signal 25 is equal to or lower than the second threshold27, the intermittent reception control circuit 5 changes a cycle ofturning ON/OFF of the power of the receiver circuit 1 according to thelevel of the reception electric field intensity signal 25 output fromthe receiver circuit 1. Specifically, the intermittent control circuit 5outputs a signal s2 for turning ON/OFF the power of the receiver circuit1 at shortened intervals as the level of the reception electric fieldintensity signal 25 is increased to a certain extent and becomes closerto the second threshold level v2. Accordingly, when the level of thereception electric field intensity signal 25 becomes high, it is judgedthat the receiving device approaches a base station and the cycle ofintermittent reception of the reception electric field intensity isshortened. Thus, delay in a start of reception electric field intensitymeasurement (start of turning ON the power of the receiver circuit 1)can be prevented.

As for judgment of the level of the reception electric field intensitysignal 25 is performed in the following manner. For example, anotherjudging circuit for judging a reception electric field intensity isprovided. Then, in the judging circuit, a reception electric fieldintensity level which is lower than the second threshold 27 is set to bea desired level and whether or not the level of the reception electricfield intensity signal s4 output from the receiver circuit 1 exceeds thedesired level is judged. Note that it is another option that a judgmentcircuit of any other electric field intensity is not provided and adesired level, in addition to the first and second threshold levels, isstored in the comparator circuit 3.

In this case, for example, as in the first embodiment, if the secondthreshold level v2 is set to be −90 dBm or more and −70 dBm or less, thedesired level may be set to be in a range from a value of 6 dB or moresmaller than v2 to a thermal noise level of the receiving device.

Thereafter, an operation of the receiving device is performed in thesame manner as in the first embodiment. Specifically, when the receivingdevice approaches the communication area and the level of the receptionfield intensity signal exceeds the second threshold 27 (continuousreception electric field intensity measurement start level v2) at thetime point t1, the comparator circuit 3 judges that the receiving deviceapproaches the communication area and the receiving device is turned tobe in a receiver circuit power ON segment 23, i.e., a continuousreception state, from an intermittent reception state (intermittentreception segment 61) and power supply voltage is supplied to thereceiver circuit 1. Thus, the reception electric field intensity v iscontinuously measured. Note that at this time point, power supply to thedemodulator circuit 2 is not started and the signal processing circuit 9and the CPU 10 each for processing a demodulation signal s1 are notoperated.

Subsequently, when the reception electric field intensity signal 25exceeds the first threshold (communication start level v1) 26 at thetime point t2, the comparator circuit 3 judges that the receiving devicehas entered the communication area, the power of the demodulator circuit2 is turned ON and the demodulator circuit 2 becomes in a demodulatorcircuit power ON segment 24. Thus, the received signal input from theantenna 7 to the demodulator circuit 2 via the receiver circuit 1 isoutput as a demodulation signal s1. The demodulation signal s1 isprocessed by the signal processing circuit 9 and then output to the CPU10. Note that in this method, the first threshold 26 is higher than thesecond threshold 27 and each of the first threshold 26 and the secondthreshold 27 is stored in the comparator circuit 3.

As described above, in the receiving device of this embodiment, thecycle of intermittent reception in the receiver circuit 1 can be changedaccording to the level of the reception electric field intensity v of areceived signal. Thus, delay in a start of reception can be reduced andalso the amount of electric current to be consumed in a standby statuswhen the receiving device is outside of a communication area can bereduced.

Fifth Embodiment

FIG. 8 is a block diagram illustrating a receiving device according to afifth embodiment of the present invention. As shown in FIG. 8, thereceiving device of this embodiment includes a receiver circuit 1 foroutputting a signal received via an antenna 7 as a reception electricfield intensity signal s4 and a demodulator circuit 2 for demodulatingthe signal processed by the receiver circuit 1 and outputting thesignal. Moreover, the receiving device of this embodiment furtherincludes a comparator circuit 3 for comparing the level of a receptionelectric field intensity signal output from the receiver circuit 1 witha first threshold and a second threshold and judging the receptionelectric field intensity signal and an intermittent reception controlcircuit 5 for outputting a signal for intermittently measuring thereception electric field intensity signal. Furthermore, the receivingdevice of this embodiment includes an operation control circuit 4 forcontrolling supply of power supply voltage to the receiver circuit 1 andthe demodulator circuit 2 and a judging circuit 8 for changing controlof the operation control circuit 4 according to a comparison result bythe comparator circuit 3. The receiving device of this embodiment alsoincludes a signal processing circuit 9 and a CPU 10 each for processinga demodulation signal s1 output from the demodulator circuit 2, and aspeed detector 31 for detecting a moving speed of the receiving deviceand outputting a detection result to the intermittent reception controlcircuit 5.

Unlike the receiving device of the fourth embodiment, the receivingdevice of this embodiment is characterized in that an intermittent cyclein an intermittent cycle signal output from the intermittent receptioncontrol circuit 5 can be set according to the moving speed of thereceiving device.

Next, the operation of the receiving device of this embodiment will bedescribed with reference to FIGS. 8 and 9. FIG. 9 is a diagramillustrating a method for controlling the receiving device of FIG. 8.Note that the description of each member also shown in FIG. 7 and detailalso described in the fourth embodiment will be omitted.

As shown in FIG. 9, when the receiving device is outside of acommunication area and also the reception electric field intensitysignal 25 is the second threshold level v2 or less, the intermittentreception control circuit 5 outputs an intermittent cycle signal so thatthe receiver circuit 1 is repeatedly turned ON/OFF in an intermittentmanner. In this case, the intermittent reception control circuit 5changes a cycle of turning ON/OFF of the receiver circuit 1 according tothe level of a speed signal output from the speed detector 31.

Specifically, when the detected moving speed of the receiving device islow (for example, before a time point t3), the operation control circuit4 outputs a signal s2 for turning ON/OFF of the receiver circuit 1 atlonger intervals. Also, when the detected moving speed of the receivingdevice is high, the operation control circuit 4 outputs a signal s2 forturning ON/OFF of the receiver circuit 1 at shorter intervals.Thereafter, the operation of the receiving device of this embodiment isperformed in the same manner as in the first embodiment and thereforethe description thereof will be omitted.

In this case, for example, when it is judged that the moving speed ofthe moving device at the time point t3 is larger than a preset movingspeed for the receiving device, the intermittent reception controlcircuit 5 outputs the signal s2 with a shorter intermittent cycle. Notethat the preset moving speed is, for example, a regulation speed.

As described above, in the receiving device of this embodiment, when themoving speed of the receiving device is low, the cycle of intermittentreception of the receiver circuit is lengthened, so that powerconsumption can be reduced by the receiver circuit. On the other hand,when the moving speed of the receiving device is high, the cycle ofintermittent reception of the receiver circuit is shortened, so thatdelay in a start of receiving a signal (receiver circuit power ON state)can be reduced.

According to the receiving method of the third embodiment, the secondthreshold is reduced to shift the receiving device to a continuousreception state at the time point t3 when the moving speed is increased.However, in the receiving method of this embodiment, unlike the thirdembodiment, an intermittent cycle in an intermittent reception state isshortened at the time point t3 when the moving speed is reduced.

Sixth Embodiment

FIG. 10 is a block diagram illustrating the configuration of a receivingdevice according to a sixth embodiment of the present invention. Asshown in FIG. 10, the receiving device of this embodiment includes areceiver circuit 1 for outputting a signal received via an antenna 7 asa reception electric filed intensity signal, a demodulator circuit 2 fordemodulating the signal processed by the receiver circuit 1 andoutputting the signal, and a comparator circuit 3 for comparing thelevel of the reception electric field intensity signal output from thereceiver circuit 1 with a first threshold and a second threshold andjudging the level of the reception electric field intensity signal. Thereceiving device of this embodiment further includes an intermittentreception control circuit 5 for outputting a signal for intermittentlymeasuring the reception electric field intensity signal, and anoperation control circuit 4 for controlling supply of power supplyvoltage to the receiver circuit 1 and the demodulator circuit 2, ajudging circuit 8 for changing control of the operation control circuit4 according to a judgment result of the comparator circuit 3, a signalprocessing circuit 9 and a CPU 10 each for processing a demodulationsignal output from the demodulator circuit 2. Furthermore, the receivingdevice of this embodiment includes a location detector 91 for measuringa location of the receiving device, a memory 92 for storing a locationof a base station, and a location comparator circuit 99 for comparing ameasurement result of the location detector 91 with the location of thebase station stored in the memory 92 and judging a distance between thereceiving device and the base station. A judgment result of the locationcomparator circuit 99 is output to the intermittent reception controlcircuit 5.

Unlike the fifth embodiment, the receiving device of this embodiment ischaracterized in that the intermittent reception control circuit 5outputs an intermittent cycle of intermittent reception of the receivingdevice according to the location of the receiving device with respect toa base station.

As the location detector 91, for example, a car navigation system usinga GPS (global positioning system) can be used. Examples of the memory 92are a car navigation system, a RAM (random access memory) and a harddisk.

Next, the operation of the receiving device of this embodiment will bedescribed with reference to FIGS. 10 and 11. FIG. 11 is a diagramillustrating a method for receiving a signal using the receiving deviceof FIG. 10.

As shown in FIG. 11, when the receiving device is outside of acommunication area and the reception filed intensity signal 25 is thesecond threshold 27, the receiver circuit 1 performs intermittentreception, thereby measuring a reception electric field intensity v of areceived signal. When measuring the electric field intensity, based on ajudgment result of the location comparator circuit 99, different cyclesof the intermittent reception are set for the case where the distancebetween the base station and the receiving device is larger than apredetermined value and the case where the distance between the basestation and the receiving device is equal to or less than thepredetermined value. Specifically, as shown in FIG. 11, when a distanceto a base station is large (before a time point t4), i.e., when thedistance between the base station and the receiving device is largerthan a predetermined setting value, as an intermittent reception segment101, an intermittent cycle is set to be relatively long. On the otherhand, when the distance to the base station is small (from the timepoint t4 to a time point t1), i.e., when the distance between the basestation and the receiving device is smaller than a predetermined settingvalue, as an intermittent reception segment 102, a cycle of intermittentreception is set to be short.

Subsequently, when the receiving device further approaches the basestation and the reception electric field intensity signal 25 exceeds thesecond threshold level v2, the comparator circuit 3 judges that thereceiving device approaches a communication area and the power of thereceiver circuit 1 is turned ON by the operation control circuit 4.Thereafter, the operation of the receiving device is performed in thesame manner as in the first embodiment.

As described above, the receiving device of this embodiment includes thememory 92 in which the location of an already known station has beenpreviously stored. Thus, the distance between the receiving device andthe base station can be precisely judged, so that power supply to thereceiver circuit can be more efficiently started without delay.Furthermore, as in each of the above-described embodiments, powerconsumption in a reception standby status when the receiving device isoutside of a communication area or the like can be reduced.

The receiving device of this embodiment is characterized in thatintermittent reception control is performed based on locationinformation of the base station stored in the memory 92. However, for anew base station of which information is not stored in the memory 92,when the new base station is found, the location of the base station ismeasured by the location detector 91 such as a GPS or the like and thenstored in the memory 92, so that the location information can be usedwhen the receiving device passes the base station next.

Seventh Embodiment

FIG. 12 is a block diagram illustrating the configuration of a receivingdevice according to a seventh embodiment of the present invention.Unlike the first embodiment, the receiving device of this embodiment ischaracterized in that the receiving device further includes a receptioncontrol circuit 111, a memory 112 and a selector 113 and is effectiveespecially when a plurality of communication frequencies received from abase station exist. As shown in FIG. 12, the receiving device of thisembodiment includes a receiver circuit 1 for outputting a signalreceived via an antenna 7 as a reception filed intensity signal s4, ademodulator circuit 2 for demodulating the signal processed by thereceiver circuit 1 and outputting the signal, a comparator circuit 3 forcomparing the level of the reception electric field intensity signal s4output from the receiver circuit 1 with a first threshold, a secondthreshold and a third threshold and judging the level of the receptionelectric field intensity signal s4. Moreover, the receiving device ofthis embodiment further includes an intermittent reception controlcircuit 5 for outputting a signal for intermittently measuring thereception electric field intensity signal and an operation controlcircuit 4 for controlling supply of power supply voltage to the receivercircuit 1 and the demodulator circuit 2. Furthermore, the receivingdevice of this embodiment includes the selector 113 for selecting acommunication frequency at which the receiver circuit 1 receives asignal when the power of the receiver circuit 1 is ON and the receptionsignal circuit 111 for selecting a plurality of communicationfrequencies in order using the selector 113 and making the comparatorcircuit 3 measure the reception electric field intensity of each of theplurality of communication frequencies. Also, the receiving device ofthis embodiment includes the memory 112 for storing a judgment result ofthe comparator circuit 3, a judging circuit 8 for changing control ofthe operation control circuit 4 according to the judgment result of thecomparator circuit 3, a signal processing circuit 9 and a CPU 10 eachfor processing a demodulation signal s1 output from the demodulatorcircuit 2.

Next, the operation of the receiving device of this embodiment will bedescribed with reference to FIGS. 12 and 13. FIG. 13 is a diagramillustrating a method for controlling the receiving device of FIG. 12.

The receiving device of this embodiment is used when a plurality ofcommunication frequencies are transmitted from a base station. In thiscase, a signal segment 20 is part of a demodulation signal s1 of aselected communication frequency in which information to a mobilestation is contained and a signal segment 21 is a part of thedemodulation signal s1 of the selected communication frequency in whichinformation to the mobile station is not contained. Moreover, in FIG.13, reception electric field intensity signals 25 and 121 show changewith time in reception field intensity v for two communicationfrequencies. In this case, a reception electric field intensity signalat a first communication frequency is assumed to be the receptionelectric field intensity signal 25 and a reception electric fieldintensity signal of a second communication frequency is assumed to bethe reception electric field intensity signal 121. In FIG. 13, as for anillustration showing the signal s2, upper part shows an output signal tothe receiver circuit 1 for receiving a first frequency signal and lowerpart shows an output signal to the receiver circuit 1 for receiving asignal of a second communication frequency.

When the receiving device approaches to a base station from the outsideof a communication area of the base station and the reception electricfield intensity v of a signal of the first communication frequency isequal to or higher than the second threshold 27, the intermittentreception control circuit 5 turns ON/OFF the power of the receivercircuit 1 at regular intervals. While the power of the receiver circuit1 is ON, the selector 113 performs selection from the plurality offrequencies in order and the comparator circuit 3 measures the receptionelectric field intensity v of a signal of each communication frequencybased on the selection of the selector 113. In this case, for example,if with the receiving device becoming closer to the base station, thereception electric field intensity of a signal of the firstcommunication frequency is increased with time, as the receptionelectric field intensity signal 25 and the reception electric fieldintensity of a signal of the second communication frequency changes asthe reception electric field intensity signal 121, the comparatorcircuit 3 judges whether or not the reception electric field intensitysignal of one of the communication frequencies which has a higherelectric field intensity v reaches the third threshold 122. If thereception electric field intensity signal of one of the communicationfrequencies which has a higher electric field intensity v is equal to orlower than the third threshold 122, intensity measurement is performedfor the respective electric field intensity signals 25 and 121 of thefirst communication frequency and the second communication frequency atthe same intervals for the same amount of time.

For example, if it is judged by the comparator circuit 3 that thereception electric field intensity v of a signal of the firstcommunication frequency exceeds the third threshold level v3 at a timepoint t5, the reception control circuit 111 stores a judgment result inthe memory 112. According to the threshold judgment result of thecomparator circuit 3 stored in the memory 112, for the firstcommunication frequency exceeding the third threshold level 122,sampling in which a time for measuring reception electric fieldintensity signal 25 is set to be a sufficient time for removing theeffects of the second communication frequency signal is performed. Forthe second communication frequency, sampling is also performed in thesame manner as that for the first communication frequency. The measuringtime is changed for each of the communication frequencies in theintermittent reception segments 123 and 124 by the reception controlcircuit 111 outputting a control signal to the intermittent receptioncontrol circuit 5.

Next, when the reception electric field intensity signal 25 of the firstcommunication frequency exceeds the second threshold level v2 at thetime point t1, the comparator circuit 3 judges that the receiving deviceapproaches a communication area and outputs a comparison result to thejudging circuit 8 and the reception control circuit 111.

In response to the output, the reception control circuit 111 makes theselector 113 select a signal of the first communication frequency, basedon the comparison result of the comparator circuit 3. The receivingdevice enters the receiver circuit power ON segment 23 from anintermittent reception state and continuously measures the receptionelectric field intensity signal 25.

Thereafter, when the reception electric field intensity signal 25 of thefirst communication frequency exceeds the first threshold level v1 atthe time point t2, the comparator circuit 3 judges that the receivingdevice has entered a communication area and outputs a comparison resultto the judging circuit 8. Then, independently from an output of theintermittent reception control circuit 5, the operation control circuit4 supplies power supply voltage to the demodulator circuit 2 and thedemodulator circuit power segment 24 begins. Thus, the signal input fromthe antenna 7 to the demodulator circuit 2 via the receiver circuit 1 isoutput as a demodulation signal s1 to the signal processing circuit 9.

With the receiving device of this embodiment, even when a plurality ofcommunication frequencies exist in a short area communication such asdedicated short range communication and a wireless LAN communication,the best communication frequency can be selected. Also, in the receivingdevice of this embodiment, power consumption in a reception standbystatus when the receiving device is outside of a communication area isreduced and, when a received signal is judged to have a predeterminedcommunication frequency, highly accurate measurement is performed over along time, so that the power of the receiver circuit can be turned ON.Therefore, communication on a desired frequency can be started withoutdelay when the receiver circuit enters a communication area. Moreover,it takes only a short time to perform measurement of the reception fieldintensities of signals of other frequencies than the desiredcommunication frequency. Thus, power can be effectively reduced.

Assume that a digital circuit is used for the receiver circuit 1 or thedemodulator circuit 2. Even when a clock signal is controlled, insteadof power, the same effect can be achieved. Thus, a current to beconsumed during a standby status can be reduced.

When the reception electric field intensity signal 25 of the firstcommunication frequency exceeds the third threshold level v3 at the timepoint t5, for the second communication frequency, it may be only judgedwhether or not a signal of the second communication frequency has beenreceived, or measurement may be performed in a segment 125 with ashorter measuring time (receiving time) than a measuring time for thefirst communication frequency. In such a case, power consumption can bereduced according to the amount of reduction in measuring time for thesignal of the second communication frequency.

FIG. 14 is an illustration showing another exemplary method forcontrolling the receiving device of this embodiment.

In the example of FIG. 14, when the reception electric field intensitysignal 25 of the first communication frequency exceeds the thirdthreshold level v3 and equal to or lower than the second threshold levelv2, a period in which only the reception electric field intensity signal25 of the first communication frequency exceeding the third thresholdlevel v3 is measured and a period in which all communication frequencies(including the first and second communication frequencies) are measuredare alternately repeated at regular intervals. That is, in this example,reception electric field intensity measurement is performed so thatmeasurement for a communication frequency (the second communicationfrequency) equal to or lower than the third threshold level v3 isskipped.

Using this method, the entire measuring time can be reduced, so thatpower consumption can be reduced.

Eighth Embodiment

FIG. 15 is a block diagram illustrating the configuration of a receivingdevice according to an eighth embodiment of the present invention.Unlike the seventh embodiment, the receiving device of this embodimentis characterized in that the reception control circuit 111 measures therate of change in reception electric field intensity and changes anintermittent reception time (receiving time) for each electric fieldintensity according to the rate of change in reception electric fieldintensity. As shown in FIG. 15, the receiving device of this embodimentincludes a receiver circuit 1 for receiving a signal via an antenna 7and outputting a reception filed intensity of the received signal as areception filed intensity signal, a demodulator circuit 2 fordemodulating the signal processed by the receiver circuit 1 andoutputting the signal, a comparator circuit 3 for comparing the level ofthe reception electric field intensity signal output from the receivercircuit 1 with a first threshold and a second threshold and judging thelevel of the reception electric field intensity signal. Moreover, thereceiving device of this embodiment further includes an intermittentreception control circuit 5 for outputting a signal for intermittentlymeasuring the reception electric field intensity signal and an operationcontrol circuit 4 for controlling supply of power supply voltage to thereceiver circuit 1 and the demodulator circuit 2. Furthermore, thereceiving device of this embodiment includes a station selector 113 forselecting a communication frequency while the power of the receivercircuit 1 is ON and outputting a selection result to the receivercircuit 1, a reception control circuit 111 for selecting a plurality ofcommunication frequencies in order using the station selector 113 andmeasuring the rate of change in reception electric field intensity ofeach of the plurality of communication frequency, and a memory 112 forstoring a measurement result of the reception control circuit 111. Also,the receiving device of this embodiment includes a judging circuit 8 forchanging control of the operation control circuit 4 according to ajudgment result of the comparator circuit 3, a signal processing circuit9 and a CPU 10 each for processing a demodulation signal output from thedemodulator circuit 2.

Next, the operation of the receiving device of this embodiment will bedescribed with reference to FIGS. 15 and 16. FIG. 16 is a diagramillustrating a method for controlling the receiving device of FIG. 15.

The receiving device of this embodiment is used when a plurality ofcommunication frequencies are transmitted from a base station. In thiscase, a signal segment 20 is part of a demodulation signal of a selectedcommunication frequency in which information to a mobile station iscontained and a signal segment 21 is part of the demodulation signal ofthe selected communication frequency in which information to the mobilestation is not contained. Moreover, in FIG. 16, reception electric fieldintensity signals 25 and 151 show change with time in the receptionfield intensity v for two communication frequencies. In this case, areception electric field intensity signal at a first communicationfrequency is the reception electric field intensity signal 25 and areception electric field intensity signal of a second communicationfrequency is the reception electric field intensity signal 151.

When the receiving device approaches to a base station from the outsideof a communication area of the base station and the reception electricfield intensity signal 25 of the first communication frequency is equalto or higher than the second threshold level v2, the intermittentreception control circuit 5 turns ON/OFF the power of the receivercircuit 1 at regular intervals. While the power of the receiver circuit1 is ON, the selector 113 performs selection from the plurality offrequencies in order and the reception control circuit 111 measures therate of change with time (a difference in terms of a unit time) inreception electric field intensity v of each of the communicationfrequencies, based on a selection made by the station selector 113. Whennone of respective reception electric field intensities of the pluralityof communication frequencies is on the increase, reception electricfield intensity measurement is performed for the first and secondcommunication frequencies at the same intervals for the same time (theintermittent reception segment 123 in FIG. 16).

When the rate of change in the reception electric field intensity signal25 of the first communication frequency is changed to be a positivevalue at the time point t5 and the rate of change in the receptionelectric field intensity signal 151 of each of other frequencies isnegative or 0, the reception control circuit 111 stores in the memory112 information that the reception electric field intensity signal 25 ofthe first communication frequency is on the increase. At this time,weighting is performed to determine, according to the rate of change inthe reception electric field intensity signal 25, which accuratemeasurement or rough measurement is to be performed and thedetermination is also stored in the memory 112. For example, assume thataccurate measurement is performed for the first communication frequencyand measurement is continuously performed for the second communication.During this period, according to a judgment result (difference judgmentresult) for the rate in the change stored in the memory 112, for thefirst frequency v of which the reception electric field intensity is onthe increase, sampling in which a time for measuring the receptionelectric field intensity v is set to be a sufficient time for removingthe effects of other frequency signals is performed and measurement forother ones of the communication frequencies (such as the secondcommunication frequency) is performed over a time which is sufficientfor judging the presence of an electric wave to be received and isshorter than a measuring time for the signal of the first communicationfrequency.

Next, when the reception electric field intensity of the signal of thefirst communication frequency exceeds the second threshold 27 at thetime point t1, the comparator circuit 3 judges that the receiving deviceapproaches a communication area and outputs a judgment result to thejudging circuit 8 and the reception control circuit 111.

In response to the output, the reception control circuit 111 makes theselector 113 select a signal of the first communication frequency, basedon the judgment result of the comparator circuit 3. The receiving deviceenters the receiver circuit power ON segment 23 from an intermittentreception state to be in a continuous reception state and continuouslymeasures the reception electric field intensity v.

Thereafter, when the reception electric field intensity v of the signalof the first communication frequency exceeds the first threshold 26 atthe time point t2, the comparator circuit 3 judges that the receivingdevice has entered a communication and outputs a judgment result to thejudging circuit 8. Thus, independently from an output of theintermittent reception control circuit 5, the operation control circuit4 supplies power supply voltage to the demodulator circuit 2 and thedemodulator circuit power segment 24 begins. Accordingly, the signalinput from the antenna 7 to the demodulator circuit 2 via the receivercircuit 1 is output as a demodulation signal to the signal processingcircuit (not shown). In this embodiment, the case where twocommunication frequencies exist has been described. However, if three ormore communication frequencies exist, signal reception and measurementcan be performed in the same manner as described above.

With the receiving device of this embodiment, control over the power ofonly necessary components of the receiving device such as the receivercircuit and the demodulator circuit is allowed in small areacommunication such as dedicated short range communication and wirelessLAN communication. Thus, even when a plurality of communicationfrequencies exist, the best communication frequency can be selected.Accordingly, in the receiving device of this embodiment, powerconsumption in a reception standby status when the receiving device isoutside of a communication area is reduced and, when a received signalis judged to have a predetermined communication frequency, highlyaccurate measurement can be performed over a long time. Moreover, ittakes only a short time to perform measurement of the reception fieldintensities of signals of other frequencies than the desiredcommunication frequency. Thus, the entire measuring time can be alsoreduced, so that power consumption can be effectively reduced.

When a digital circuit is used for the receiver circuit or thedemodulator circuit, even if a clock signal is controlled, instead ofpower supply source, the same effect can be achieved and powerconsumption can be reduced. Moreover, when the reception electric fieldintensity of signal of one of communication frequencies has turned toincrease, instead of reducing the measuring time for other ones of thecommunication frequencies, the cycle of electric field intensitymeasurement for other ones of the communication frequencies may belengthened.

Ninth Embodiment

FIG. 17 is a block diagram illustrating the configuration of a receivingdevice according to a ninth embodiment of the present invention. Asshown in FIG. 17, the receiving device of this embodiment includes areceiver circuit 1 for receiving a signal via an antenna 7 andoutputting a reception filed intensity of the received signal as areception filed intensity signal and a demodulator circuit 2 fordemodulating the signal processed by the receiver circuit 1 andoutputting the signal. Moreover, the receiving device of this embodimentfurther includes a comparator circuit 3 for comparing the level of thereception electric field intensity signal output from the receivercircuit 1 with a first threshold and a second threshold and judging thelevel of the reception electric field intensity signal and anintermittent reception control circuit 5 for outputting a signal forintermittently measuring the reception electric field intensity signal.Furthermore, the receiving device of this embodiment includes anoperation control circuit 4 for controlling supply of power supplyvoltage to the receiver circuit 1 and the demodulator circuit 2, astation selector 113 for selecting a communication frequency while thepower of the receiver circuit 1 is ON, and a location detector 161 formeasuring the location of the receiving device. The receiving device ofthis embodiment also includes a memory 112 for storing the location of abase station and the communication frequency and a reception controlcircuit 111 for making the station selector 113 selects a communicationfrequency. The receiving device includes a location comparator circuit120 for comparing an output of the location detector 161 with locationinformation from the memory 112 and judging a distance between thereceiving device and a base station, a judging circuit 8 for changingcontrol of the operation control circuit 4 according to a judgmentresult of the comparator circuit 3, a signal processing circuit 9 and aCPU 10 each for processing a demodulation signal output from thedemodulator circuit 2. Note that the location detector 161 is, forexample, a car navigation system or the like in which a GPS system ismounted and is the same as the location detector 91 of the sixthembodiment.

Next, the operation of the receiving device of this embodiment will bedescribed with reference to FIGS. 17 and 18. FIG. 18 is a diagramillustrating a method for controlling the receiving device of FIG. 17.

The receiving device of this embodiment is used when a plurality ofcommunication frequencies exist. In this case, the location comparatorcircuit 120 measures the location of the receiving device by thelocation detector 161, compares the measurement result with an output ofthe memory 112 in which the location of a base station and acommunication frequency have been previously stored, and judges adistance between the receiving device and the base station. The judgmentresult is input to the reception control circuit 111. If the locationcomparator circuit 120 judges that the receiving device is located farfrom the base station and the reception electric field intensity signal25 is equal to or lower than the second threshold level v2, theintermittent reception control circuit 5 performs intermittent receptionin an intermittent reception segment 123. In this case, simple receptionelectric field intensity measurement which is sufficient for judging thepresence of an electric wave to be received is performed for all thecommunication frequencies in each ON segment.

Next, when the receiving device approaches the base station and thecomparator circuit 3 judges that the reception electric field intensityof a signal of a communication frequency of the base station is equal toor lower than the second threshold, the reception control circuit 111selects the communication frequency of the base station stored in thememory 112 using the station selector 113 and performs sampling in whicha time for measuring reception electric field intensity is set to be asufficient time for removing the effects of other frequency signals.Measurement for other ones of the communication frequencies is performedover a time which is sufficient for judging the presence of an electricwave to be received and is shorter than a measuring time for the signalof the communication frequency stored in the memory 112.

Subsequently, when the receiving device further approaches the basestation and the reception electric field intensity signal 25 exceeds thesecond threshold level v2 at the time point t1, the comparator circuit 3judges that the receiving device approaches a communication area, thereception control circuit 111 selects a communication frequencyexceeding the second threshold level v2 using the station selector 113,and the power of the receiving circuit 1 is kept continuously ON by theoperation control circuit 4. Then, the receiving device enters thereceiver circuit power ON segment (continuous reception segment) 23 froman intermittent reception state to be in a continuous reception stateand continuously measures the reception electric field intensity.

Thereafter, when the reception electric field intensity signal 25exceeds the first threshold level v1 at the time point t2, thecomparator circuit 3 judges that the receiving device has entered acommunication area and outputs a judgment result to the judging circuit8. Accordingly, the power of the demodulator circuit 2 is turned ON bythe operation control circuit 4 and the demodulator circuit power ONsegment 24 begins. Thus, the received signal input into the demodulatorcircuit 2 from the antenna 7 via the receiver circuit 1 is output as ademodulated circuit.

With the receiving device of this embodiment, even when a plurality ofcommunication frequencies exist in a short area communication such asdedicated short range communication and a wireless LAN communication,the best communication frequency can be efficiently selected using thelocation of a base station and information for communication frequenciespreviously stored in the memory.

Furthermore, in the receiving device of this embodiment, highly accuratemeasurement can be performed in a reception standby status when thereceiving device is outside of a communication area, over a long time,and for the communication frequency of a base station which thereceiving device approaches, and measurement can be performed for otherfrequencies, in a simple manner, and over a short time. Thus, the entiremeasuring time can be also reduced, so that power consumption can bereduced. Moreover, power is supplied to only necessary components of thereceiving device such as the receiver circuit and the demodulatorcircuit, thereby reducing power consumption.

The receiving device of this embodiment is characterized in thatintermittent reception control is performed to a signal of thecommunication frequency of a base station previously stored in thememory 112. For a base station of which information is not stored in thememory 112, the location detector 161 measures the location of a newbase station when the base station is found and stores with itscommunication frequency in the memory 112. Thus, the stored informationcan be used when the receiving device passes the base station next.

Tenth Embodiment

FIG. 19 is a diagram illustrating a communication semiconductorintegrated circuit including a receiving device according to a tenthembodiment of the present intention. That is, FIG. 19 shows a circuitdiagram of the receiving device of the present invention. Note that eachmember also described in the above-described embodiments is identifiedby the same reference numeral and therefore the detail descriptionthereof will be omitted. FIG. 19 shows a specific example for each ofthe receiver circuit 1, the demodulator circuit 2, the comparatorcircuit 3, the intermittent receiver circuit 5, the operation controlcircuit 4, the judging circuit 8, the signal processing circuit 9 andthe CPU 10 described in each of the above-described embodiment.

The communication semiconductor integrated circuit of this embodimentincludes any one of the receiving devices of the first through ninthembodiments of the present invention integrated and provided on asemiconductor chip. Specifically, in the semiconductor integratedcircuit of this embodiment for use in a receiving device, all or some ofthe receiver circuit 1, the demodulator circuit 2, the comparatorcircuit 3, the intermittent control circuit 5, the operation controlcircuit 4, the judging circuit 8, the signal processing circuit 9 andthe CPU 10 are integrated on a semiconductor substrate.

Thus, a device for transmitting/receiving information to/from a basestation may be integrated on a single chip.

Note that, for example, the transmitting/receiving IC of FIG. 19 and acontrol section can be integrated on another semiconductor chip.

Eleventh Embodiment

A transmitting/receiving device according to an eleventh embodiment ofthe present invention is obtained by implementing on a circuit substrateany one of the receiving devices of the first through ninth embodimentsof the present invention, a base band circuit (signal processingcircuit) for processing a modulation signal which has been received orgenerating a transmission modulation signal, and a transmitting devicefor generating a transmission signal based on the transmissionmodulation signal.

The transmitting/receiving device of this embodiment is small and can beincorporated in a wireless communication system in a simple manner. Thetransmitting/receiving device can be also made to be in a receptionstandby state at a low consumption current when being outside of acommunication area, so that interactive communication between the deviceand a base station (roadside wireless communication system).

The transmitting/receiving device has the same effects even if thedevice has a structure in which the receiving device, the base bandcircuit and the transmitting device are mounted on separate substrates,respectively.

Twelfth Embodiment

A transmitting/receiving device according to a twelfth embodimentincludes any one of the receiving devices of the first through ninthembodiments, a base band circuit (signal processing circuit) forprocessing a received modulation signal or generating a transmissionmodulation signal, a transmitting device for generating a transmissionsignal based on the transmission modulation signal, and an antenna.Moreover, the transmitting/receiving device may further include a CPU.With the configuration, interactive communication between the device anda base station (roadside wireless communication system) becomes possibleand power consumption when the device is outside of a communication areacan be reduced.

In such a transmitting/receiving device, when high-speed andlarge-volume data exchange is necessary, for example, in paying a tollusing ETC system, in obtaining information using DSRC, in interactivelyexchanging information with GPS, or in paying at a gas station,information can be exchanged between a base station and a movingstation.

Thirteenth Embodiment

FIG. 20 is an illustration of a transport apparatus according to athirteenth embodiment of the present invention. The transport apparatusincludes any one of the receiving devices (for example, an ETC system tobe mounted in a passenger car) of the above-described embodiments beingmounted therein.

As shown in FIG. 20, the transport apparatus of this embodiment includesthe transmitting/receiving device of the twelfth embodiment, a powersupply device, and an antenna. A vehicle speed detection devicecorresponds to, for example, the speed detector 31 of FIG. 4 and may beprovided outside of the receiving device.

With this configuration, even when the transport apparatus moves, delayin start of receiving a signal is reduced and a receiving circuitperforms intermittent reception when the transport apparatus is out ofcommunication area. Thus, power consumption in communication standbystate can be reduced to a lower level than that of a known transportapparatus.

If, for example, the receiving device of the present invention is usedfor a DSRC system including seven communication frequencies, control canbe performed such that for a frequency having a small reception electricfield intensity (i.e., a signal for transmitting unnecessary informationat a predetermined place), an intermittent interval becomes long and areceiving time becomes short. Thus, excessive power consumption can bereduced.

Fourteenth Embodiment

FIGS. 21A and 21B are illustrations describing a transportable wirelesscommunication system according to a fourteenth embodiment of the presentinvention.

As shown in FIG. 21A, the transportable wireless communication systemincludes the transmitting/receiving device having an antenna anddescribed in the eleventh embodiment and a battery functioning as apower source for external transmitting/receiving device.

FIG. 21B is an enlarged view of a transmitting/receiving device. Thetransmitting/receiving device includes control buttons for controlling afunction menu displayed on a display screen, a card insertion slot intowhich a card such as a credit card is inserted, a card ejection button,a liquid crystal display screen for displaying information received froma base station, a forced reception operation button for manually turningON the power of the receiving device, an error display section fordisplaying a communication error with a base station, and a standbydisplay section for indicating that the receiving device is in a statein which communication with a base station can be started. By pressingthe forced reception operation button, even when the level of anelectric field intensity signal is not over the second threshold, thereceiving device can be switched from an intermittent reception state toa continuous reception state. Therefore, when the attitude of thereceiving device is not stable and the receiving device can not receivea reception electric filed intensity because the receiving device istransportable, the situation where start of receiving a signal isdelayed as the receiving device approaches a desired base station can beavoided.

With this configuration, a transmitting/receiving device according tothe present invention can be used for other applications than anapplication to a transmitting/receiving device for a vehicle and a rangeof use of a transmitting/receiving device according to the presentinvention can be widened. For example, the transportable wirelesscommunication system can be taken along anywhere and thus, for example,is capable of receiving information such as interpretation transmittedin a theme park. Also, communication can be established in any place andany occasion such as at the goal in a marathon for checking runners'results. In a receiving device according to the present invention, eachcircuit is operated at a time point where the operation of the circuitis required, thus allowing reduction in power consumption. Therefore,the portable wireless communication system can be operated for a longtime on a single charge of a battery. Moreover, when a function menu isdisplayed on a display, a user can select an item from the menu todisplay information and make a payment with a credit card.

Fifteenth Embodiment

A receiving device according to the present invention is useful in awireless communication system. Use of any one of the receiving devicesof the first through ninth embodiment of the present invention for abase station and moving station allows wireless interactivecommunication between the base station and the moving station. In thecase where neither a base station nor a moving station is within acommunication possible area, intermittent reception is performed ineither one of the base station and the moving station so as to make thereceiving device in a communication standby state. Thus, powerconsumption can be reduced.

Sixteenth Embodiment

FIG. 22 is an illustration of an exemplary communication systemaccording to a sixteenth embodiment of the present invention. As shownin FIG. 22, the communication system of this embodiment includes any oneof the receiving devices of the first through ninth embodiments of thepresent invention, an error display section for displaying acommunication error with a base station, a standby display section forindicating that communication with a base station is possible, and aspeaker for issuing warning. By inserting a card (not shown) as anexternal component into a card insertion slot, communication with thebase station becomes possible. For example, if the level of an electricfield intensity signal exceeds the second threshold without a cardinserted in the slot, the standby display section flashes or the speakerissues warning to urge insertion of a card. Thus, in a communicationpossible area with a base station, a communication error between thecommunication system and a base station can be prevented.

Furthermore, as in the fourteenth embodiment, the communication systemof this embodiment may include a forced reception operation button forturning ON the power of the receiver circuit to allow communication witha base station.

In each of the above-described embodiments, the case where the power ofthe receiving circuit 1 is turned ON at the time point t1 where thereception electric field intensity signal 25 exceeds the secondthreshold level v2 has been described. However, the power of thereceiving circuit 1 can be turned ON at any time at least during aperiod between the time point t1 and the time point t2 and the presentinvention is realized as long as the power of the receiving circuit 1 isON without delay at the time point t2 which indicates that the movingstation enters a communication area.

Note that in each of the drawings according to the above-describedembodiments, the case where the signal segment 20, i.e., part of areceived demodulation signal in which information to the mobile stationis contained and the signal segment 21, i.e., part of the receivedsignal in which information to the mobile station is not contained arealternately repeated with certain intervals has been described. However,it is needless to say that the present invention is not limited thereto.Also, the case where as soon as the moving station enters acommunication area, a demodulation signal is output from the signalsegment 20 has been described. However, a demodulation signal may beoutput at a time point where the signal segment 21 begins and, needlessto say, a demodulation signal may be output with delay.

The receiving device of the present invention described above ispreferably used for a communication system used in small areacommunication, represented by dedicated short range communication andwireless LAN communication.

1. A receiving device for receiving a signal transmitted from a basestation, the device comprising: a receiver circuit for outputting areception electric field intensity of a signal received via an antennaas a reception electric field intensity signal; a comparator circuit forcomparing a level of the reception electric field intensity signaloutput from the receiver circuit using a first threshold indicating thatthe receiving device is in a communication area and a second thresholdwhich is lower than the first threshold and indicates a continuous fieldmeasurement level and outputting a result of the comparison; anintermittent reception control circuit for outputting a periodic signalfor intermittently operating the receiver circuit; and an operationcontrol circuit for controlling an operation of the receiver circuitbased on a comparison result of the comparator circuit, wherein, basedon the comparison result of the comparator circuit, if the level of thereception electric field intensity signal is lower than the secondthreshold, the operation control circuit makes the receiver circuitperform intermittent operation based on the periodic signal and if thelevel of the reception electric field intensity signal is equal to orhigher than the second threshold, the operation control circuit makesthe receiver circuit perform a continuous operation.
 2. The receivingdevice of claim 1, wherein the receiver circuit further includes thefunction of amplifying and outputting the received signal, wherein thereceiving device further includes a demodulator circuit for demodulatingthe amplified signal and outputting the signal as a modulation signal,and wherein the operation control circuit controls an operation of thedemodulator circuit.
 3. The receiving device of claim 2, wherein, basedon the comparison result of the comparator circuit, the operationcontrol circuit operates the demodulator circuit when the level of thereception electric field intensity signal exceeds the first threshold.4. The receiving device of claim 2, wherein, based on the comparisonresult of the comparator circuit, the operation control circuit suppliesa first clock signal to the demodulator circuit when the level of thereception electric field intensity signal exceeds the first threshold.5. The receiving device of claim 2, further comprising: a signalprocessing circuit for processing the demodulation signal output fromthe demodulator circuit; and a processing unit for arithmeticallyprocessing the demodulation signal processed by the signal processingcircuit, wherein, based on the comparison result of the comparatorcircuit, the operation control circuit supplies a second clock signal tothe signal processing circuit and the processing unit to control thesignal processing circuit and the processing unit when the level of thereception electric field intensity signal is lower than the secondthreshold.
 6. The receiving device of claim 1, wherein the secondthreshold has a first boundary value which is smaller than the secondthreshold and a second boundary value which is larger than the secondthreshold, wherein an operation of the receiver circuit is changed froman intermittent operation to a continuous operation when the level ofthe reception electric field intensity signal is increased to exceed thesecond boundary value, and wherein the operation of the receiver circuitis changed from a continuous operation to an intermittent operation whenthe level of the reception electric field intensity signal is reduced tobe equal to the first boundary value.
 7. The receiving device of claim1, further comprising a speed detector for detecting a moving speed ofthe receiving device and outputting the detected speed as a speedsignal, wherein the comparator circuit changes the second thresholdaccording the speed signal.
 8. The receiving device of claim 7, whereinwhen the moving speed of the receiving device is increased, thecomparator circuit changes the second threshold so that a differencebetween the first threshold and the second threshold becomes larger. 9.The receiving device of claim 1, wherein the intermittent receptioncontrol circuit changes the periodic signal according to the receptionelectric field intensity signal.
 10. The receiving device of claim 9,wherein when the level of the reception electric field intensity signalbecomes high, the intermittent control circuit shortens a cycle of theperiodic signal.
 11. The receiving device of claim 1, further comprisinga speed detector for detecting a moving speed of the receiving deviceand outputting the detected speed as a speed signal, wherein theintermittent reception control circuit changes the periodic signalaccording to the speed signal.
 12. The receiving device of claim 11,wherein when the moving speed of the receiving device is increased, theintermittent reception control circuit shortens a cycle of the periodicsignal.
 13. The receiving device of claim 1, further comprising: amemory for storing location information for the base station; and alocation detector for detecting a location of the receiving device,wherein the intermittent reception control circuit changes the periodicsignal according to a result of comparison between the location of thebase station stored in the memory and the result of detection by thelocation detector.
 14. The receiving device of claim 13, wherein theintermittent reception control circuit shortens a cycle of the periodicsignal when a distance between the base station and the receiving deviceis decreased.
 15. The receiving device of claim 1, further comprising: areception control circuit for controlling a receiving time which ittakes for the receiver circuit to receive the signal via the antenna,wherein the comparator circuit compares a level of the receptionelectric intensity signal with the first threshold, the secondthreshold, and a third threshold which is lower than the secondthreshold and indicating whether or not the signal to be receivedexists, and wherein, as the comparison result of the comparator circuit,when the level of the reception electric field intensity signal exceedsthe third threshold, the reception control circuit increases thereceiving time so that the receiving time becomes longer than thereceiving time when the level of the reception electric field intensitysignal is equal to or lower than the third threshold.
 16. The receivingdevice of claim 15, wherein the intermittent reception control circuitincreases a cycle of the periodic signal when the level of the receptionelectric field intensity signal is equal to or lower than the thirdthreshold.
 17. The receiving device of claim 1, further comprising areception control circuit for measuring a rate of change in thereception electric field intensity, wherein the reception controlcircuit changes, according to the rate of change, a receiving time whichit takes for the receiver circuit to receive the signal.
 18. Thereceiving device of claim 1, further comprising: a reception controlcircuit for controlling a receiving time which it takes for the receivercircuit to receive the signal; a memory for storing a location of thebase station and frequency information of the signal transmitted fromthe base station; and a location detector for detecting a location ofthe receiving device, wherein the reception control circuit changes thereceiving time according to a result of comparison between the locationof the base station stored in the memory and a detection result of thelocation detector.
 19. The receiving device of claim 18, wherein when adistance between the base station and the receiving device is reduced,the receiving time is increased.
 20. The receiving device of claim 18,further comprising a station selector for selecting, from signals of aplurality of frequencies, a frequency signal which the receiver circuitis to receive, based on an output of the reception control circuit,wherein, as the result comparison of the comparator circuit, the stationselector selects from the plurality of frequency signals a frequencysignal having a reception electric field intensity level exceeding thesecond threshold and the receiver circuit receives the selectedfrequency signal.
 21. The receiving device of claim 1, wherein the firstthreshold is in a range of −70 dBm or more and −60 dBm or less, andwherein the second threshold is in a range from −90 dBm or more and −70dBm or less.
 22. A semiconductor integrated circuit comprising: asemiconductor substrate; and the receiving device of claim 1 integratedon the semiconductor device.
 23. A transmitting/receiving devicecomprising: the receiving device of claim 1; a transmitting device foroutputting a signal processed by the receiving device to the basestation; and an antenna for receiving a signal transmitted from the basestation, outputting the received signal to the receiving circuit, andalso outputting the signal output from the transmitting device to thebase station.
 24. The transmitting/receiving device of claim 23, furthercomprising a notifying section for notifying, the comparison result ofthe comparator circuit, that the level of the reception electric fieldintensity signal has exceeded the second threshold.
 25. A transportapparatus comprising: the transmitting/receiving device of claim 23; anda power supply device for supplying power to the transmitting/receivingdevice.
 26. A portable transmitting/receiving device comprising: thetransmitting/receiving device of claim 23; and a battery for supplyingpower to the transmitting/receiving device.
 27. A communication systemcomprising; the receiving device of claim 1; and a transmitting devicefor transmitting a signal processed by the receiving device the basestation.
 28. A method for receiving a signal using a receiving devicefor receiving a signal transmitted from a base station, the methodcomprising: a first step of receiving the signal via an antenna by areceiver circuit; a second step of outputting a reception electric fieldintensity of the received signal as a reception electric field intensitysignal from the receiver circuit; a third step of comparing the level ofthe reception electric field intensity signal with a first thresholdindicating that the receiving device is in a communication area and asecond threshold which is lower than the first threshold and indicates acontinuous field measurement start level; a fourth step of making, ifthe level of the reception electric field intensity signal is lower thanthe second threshold, as a result of the comparison, the receivercircuit perform an intermittent operation; and a fifth step of making,if the level of the reception electric field intensity signal is equalto or higher than the second threshold, as a result of the comparison,the receiver circuit perform a continuous operation.
 29. The method ofclaim 28, further comprising a sixth step of making, if the level of thereception electric field intensity signal is equal to or higher than thefirst threshold, as a result of the comparison, a demodulator circuitfor demodulating the signal received by the receiver circuit perform anoperation.
 30. The method of claim 29, wherein in the sixth step, aclock signal for demodulation operation is given to the demodulatorcircuit.
 31. The method of claim 28, wherein each of the fourth step andthe fifth step includes the step of giving a clock signal to a signalprocessing circuit for processing a signal output from the receivercircuit and a processing unit for arithmetically processing the signal.32. The method of claim 28, wherein the third step includes the step ofcomparing the level of the reception electric field intensity signalwith the second threshold by hysterisis control using a first boundaryvalue which is lower than the second threshold and a second boundaryvalue which is higher than the second threshold.
 33. The method of claim28, further comprising before the third step: a seventh step ofdetecting a moving speed of the receiving device and outputting themoving speed as a speed signal; and an eighth step of changing thesecond threshold according to the speed signal.
 34. The method of claim33, wherein in the eighth step, when the moving speed is increased, thesecond threshold is changed so that a difference between the firstthreshold and the second threshold is increased.
 35. The method of claim28, wherein the fourth step includes a ninth step of changing a cycle ofthe intermittent operation according to the reception electric fieldintensity signal.
 36. The method of claim 35, wherein in the ninth step,when the level of the reception electric field intensity signal is high,the cycle of the intermittent operation is shortened.
 37. The method ofclaim 28, further comprising: a tenth step of detecting a moving speedof the receiving device and outputting the detected moving speed as aspeed signal; and an eleventh step of changing a cycle of theintermittent operation according to the speed signal.
 38. The method ofclaim 37, wherein in the eleventh step, when the moving speed isincreased, the cycle of the intermittent operation is shortened.
 39. Themethod of claim 28, further comprising: the step of detecting a locationof the receiving device; and the step of calculating a distance betweena location of the base station stored in the receiving device and thedetected location of the receiving device, wherein the fourth stepfurther includes a twelfth step of changing a cycle of the intermittentoperation according to the calculated distance.
 40. The method of claim39, wherein in the twelfth step, when the distance between the basestation and the receiving device is reduced, the cycle of theintermittent operation is shortened.
 41. The method of claim 28, whereinthe third step further includes the step of comparing the level of thereception electric field intensity signal with a third threshold whichindicates whether or not a signal to be received exists is lower thanthe second threshold and, wherein the fourth step includes the step ofincreasing, if the level of the reception electric field intensitysignal exceeds the third threshold, as a result of the comparison, areceiving time of the receiver circuit so that the receiving time islonger than a receiving time when the level of the reception electricfield intensity signal is equal to or lower than the third threshold.42. The method of claim 28, wherein the fourth step includes the step ofchanging a receiving time which it takes for the receiver circuit toreceive the signal, according to a rate of change in the receptionelectric field intensity.
 43. The method of claim 28, furthercomprising: the step of detecting a location of the receiving device;and the step of calculating a distance between a location of the basestation stored in the receiving device and the detected location of thereceiving device, wherein the fourth step includes the step of changinga receiving time which it takes for the receiver circuit to receive thesignal according to the calculated distance.
 44. The method of claim 28,further comprising, after the third step, the step of selecting, fromthe plurality of received signals, a signal with a reception electricfield intensity signal having a level equal to or higher than the secondthreshold.
 45. The method of claim 28, wherein the fifth step includesthe step of notifying that the level of the reception electric fieldintensity signal is equal to or higher than the second threshold.